Remove the need to invoke mknod using FFI.

This removes fpm's direct use of FFI and removes `ffi` as a direct
dependency. For #1795

Cases:
* A unix socket.
* A named pipe
* A charDev should now fail (like /dev/tty)
* A blockDev should now fail (like /dev/sda1)

NOTE: In this change, chardev and blockdev support have been removed.
These "copies" previously were just calling `mknod` with identical
mode, basically copying the `mode` from stat(2) to mknod(2).
Exceptions are now thrown for chardev and blockdev.

Test cases:

    # Try to package a named pipe.
    % mkfifo /tmp/z.pipe
    % bundle exec bin/fpm -s dir -t rpm -n example /tmp/z.pipe
    Created package {:path=>"example-1.0-1.x86_64.rpm"}

    % rpm -qlvp example-1.0-1.x86_64.rpm
    prw-rw-r--    1 root     root                        0 Jun 17 22:40 /tmp/z.pipe

    # Create the unix socket
    % nc -lU /tmp/z.sock

    # Package it into an rpm
    % bin/fpm -s dir -t rpm -n example /tmp/z.sock |& less
    {:timestamp=>"2021-06-17T22:33:27.780347-0700", :message=>"Created package", :path=>"example-1.0-1.x86_64.rpm"}

    # Verify the file is of socket type ('s' at beginning of file mode
    % rpm -qlvp example-1.0-1.x86_64.rpm
    srwxrwxr-x    1 root     root                        0 Jun 17 22:33 /tmp/z.sock

fpm.gemspec

@@ -46,11 +46,6 @@ Gem::Specification.new do |spec|
   # Ref: https://github.com/jordansissel/fpm/issues/1592
   spec.add_dependency("childprocess", "< 1.0.0") # license: ???
 
-  # For calling functions in dynamic libraries
-  # This is pinned because 1.13.0 requires Ruby 2.3 or later
-  # Ref: https://github.com/jordansissel/fpm/issues/1708
-  spec.add_dependency("ffi", "~> 1.12.0") # license: GPL3/LGPL3
-
   spec.add_development_dependency("rake", "~> 10") # license: MIT
 
   # For creating FreeBSD package archives (xz-compressed tars)

lib/fpm/util.rb

@@ -1,28 +1,22 @@
 require "fpm/namespace"
 require "childprocess"
-require "ffi"
 require "fileutils"
 
 # Some utility functions
 module FPM::Util
-  extend FFI::Library
-  ffi_lib FFI::Library::LIBC
-
-  # mknod is __xmknod in glibc a wrapper around mknod to handle
-  # various stat struct formats. See bits/stat.h in glibc source
-  begin
-    attach_function :mknod, :mknod, [:string, :uint, :ulong], :int
-  rescue FFI::NotFoundError
-    # glibc/io/xmknod.c int __xmknod (int vers, const char *path, mode_t mode, dev_t *dev)
-    attach_function :xmknod, :__xmknod, [:int, :string, :uint, :pointer], :int
-  end
 
   # Raised if safesystem cannot find the program to run.
   class ExecutableNotFound < StandardError; end
 
   # Raised if a safesystem program exits nonzero
   class ProcessFailed < StandardError; end
 
+  # Raised when a named pipe cannot be copied due to missing functions in fpm and ruby.
+  class NamedPipeCannotBeCopied < StandardError; end
+
+  # Raised when an attempting to copy a special file such as a block device.
+  class UnsupportedSpecialFile < StandardError; end
+
   # Is the given program in the system's PATH?
   def program_in_path?(program)
     # return false if path is not set
@@ -308,19 +302,6 @@ def tar_cmd_supports_sort_names_and_set_mtime?
     return @@tar_cmd_deterministic
   end
 
-  # wrapper around mknod ffi calls
-  def mknod_w(path, mode, dev)
-    rc = -1
-    case %x{uname -s}.chomp
-    when 'Linux'
-      # bits/stat.h #define _MKNOD_VER_LINUX  0
-      rc = xmknod(0, path, mode, FFI::MemoryPointer.new(dev))
-    else
-      rc = mknod(path, mode, dev)
-    end
-    rc
-  end
-
   def copy_metadata(source, destination)
     source_stat = File::lstat(source)
     dest_stat = File::lstat(destination)
@@ -346,10 +327,21 @@ def copy_metadata(source, destination)
 
   def copy_entry(src, dst, preserve=false, remove_destination=false)
     case File.ftype(src)
-    when 'fifo', 'characterSpecial', 'blockSpecial', 'socket'
-      st = File.stat(src)
-      rc = mknod_w(dst, st.mode, st.dev)
-      raise SystemCallError.new("mknod error", FFI.errno) if rc == -1
+    when 'fifo'
+      if File.respond_to?(:mkfifo)
+        File.mkfifo(dst)
+      elsif program_exists?("mkfifo")
+        safesystem("mkfifo", dst)
+      else
+        raise NamedPipeCannotBeCopied("Unable to copy. Cannot find program 'mkfifo' and Ruby is missing the 'File.mkfifo' method: #{src}")
+      end
+    when 'socket'
+      require "socket"
+      # In 2019, Ruby's FileUtils added this as a way to "copy" a unix socket.
+      # Reference: https://github.com/ruby/fileutils/pull/36/files
+      UNIXServer.new(dst).close()
+    when 'characterSpecial', 'blockSpecial'
+      raise  UnsupportedSpecialFile.new("File is device which fpm doesn't know how to copy (#{File.ftype(src)}): #{src}")
     when 'directory'
       FileUtils.mkdir(dst) unless File.exists? dst
     else