Auto merge of #2066 - wmanley:preadv2-pwritev2, r=JohnTitor

Linux: Add `preadv2` and `pwritev2` and associated constants

These functions are the same as `preadv` and `pwritev` but have a flags
parameter.  `preadv2()` and `pwritev2()` first appeared in Linux 4.6.
Library support was added in glibc 2.26.

See the definition of the constants in [linux/fs.h](https://github.com/torvalds/linux/blob/fcadab740480e0e0e9fa9bd272acd409884d431a/tools/include/uapi/linux/fs.h#L288-L301).

src/unix/linux_like/linux/gnu/mod.rs

@@ -862,6 +862,15 @@ pub const EPOLLWAKEUP: ::c_int = 0x20000000;
 pub const SEEK_DATA: ::c_int = 3;
 pub const SEEK_HOLE: ::c_int = 4;
 
+// linux/fs.h
+
+// Flags for preadv2/pwritev2
+pub const RWF_HIPRI: ::c_int = 0x00000001;
+pub const RWF_DSYNC: ::c_int = 0x00000002;
+pub const RWF_SYNC: ::c_int = 0x00000004;
+pub const RWF_NOWAIT: ::c_int = 0x00000008;
+pub const RWF_APPEND: ::c_int = 0x00000010;
+
 // linux/rtnetlink.h
 pub const TCA_PAD: ::c_ushort = 9;
 pub const TCA_DUMP_INVISIBLE: ::c_ushort = 10;
@@ -1423,6 +1432,20 @@ extern "C" {
         dirfd: ::c_int,
         path: *const ::c_char,
     ) -> ::c_int;
+    pub fn preadv2(
+        fd: ::c_int,
+        iov: *const ::iovec,
+        iovcnt: ::c_int,
+        offset: ::off_t,
+        flags: ::c_int,
+    ) -> ::ssize_t;
+    pub fn pwritev2(
+        fd: ::c_int,
+        iov: *const ::iovec,
+        iovcnt: ::c_int,
+        offset: ::off_t,
+        flags: ::c_int,
+    ) -> ::ssize_t;
 }
 
 extern "C" {