Fix require failing in Termux on Android

Note that this change does not make rbtrace *actually* work in Termux - it just allows an app to start up without an exception when it has `require 'rbtrace'`.

The exception is:

```
$ ruby -r rbtrace -e puts
Traceback (most recent call last):
        1: from /data/data/com.termux/files/usr/lib/ruby/site_ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require'
/data/data/com.termux/files/usr/lib/ruby/site_ruby/2.6.0/rubygems/core_ext/kernel_require.rb:54:in `require': cannot load such file -- rbtrace (LoadError)
        2: from /data/data/com.termux/files/usr/lib/ruby/site_ruby/2.6.0/rubygems/core_ext/kernel_require.rb:34:in `require'
        1: from /data/data/com.termux/files/usr/lib/ruby/site_ruby/2.6.0/rubygems/core_ext/kernel_require.rb:130:in `rescue in require'
/data/data/com.termux/files/usr/lib/ruby/site_ruby/2.6.0/rubygems/core_ext/kernel_require.rb:130:in `require': dlopen failed: cannot locate symbol "rindex" referenced by "/data/data/com.termux/files/usr/lib/ruby/gems/2.6.0/gems/rbtrace-0.4.13/lib/rbtrace.so"... - /data/data/com.termux/files/usr/lib/ruby/gems/2.6.0/gems/rbtrace-0.4.13/lib/rbtrace.so (LoadError)
```

It turns out that Android (9, at least) does not include `rindex` - I believe because it has been [deprecated](https://linux.die.net/man/3/rindex) for many years; `strrchr` is apparently a drop-in replacement.

After that, there was another exception about the `SUN_LEN` symbol missing. I added [this definition](https://stackoverflow.com/a/12628469) for the macro. That then got the gem able to be required without error.

I would have liked to get rbtrace working in Termux, but this is as far as I've been able to progress for now. I'm having trouble with the `ipcs` binary. It's [deliberately not provided by Termux's `util-linux` package](https://github.com/termux/termux-packages/blob/8076aec21f8d4b0dd5f880af1af467961dcbbbbb/packages/util-linux/build.sh#L19) for some reason. With root, I can run Stericson Busybox's `ipcs`, but it just returns this:

```
$ su -c /sbin/ipcs

kernel not configured for message queues

kernel not configured for shared memory

kernel not configured for semaphores
```

I'm hoping that's not just a quirk of the particular kernel on [my device](https://www.indiegogo.com/projects/cosmo-communicator). But perhaps the shared memory functionality is not actually suppported on Android? I don't yet know. Anyway.

`test.sh` does not work since rbtrace itself does not yet work on Android. The tmpdir change in there gets it one step closer at least - `/tmp` does not exist on Android.

This commit was tested by simply:

```bash
$ gem build rbtrace.gemspec && gem install rbtrace-0.4.13.gem && ruby -r rbtrace -e puts
```

ext/rbtrace.c

@@ -51,6 +51,11 @@
 #endif
 
 
+// The SUN_LEN macro is not available on Android
+#ifndef SUN_LEN
+#define SUN_LEN(ptr) ((size_t) (((struct sockaddr_un *) 0)->sun_path) + strlen((ptr)->sun_path))
+#endif
+
 static uint64_t
 ru_utime_usec()
 {
@@ -688,7 +693,7 @@ rbtracer_add(char *query, bool is_slow)
       *idx = NULL,
       *method = NULL;
 
-    if (NULL != (idx = rindex(query, '.'))) {
+    if (NULL != (idx = strrchr(query, '.'))) {
       klass_begin = 0;
       klass_end = idx - query;
       is_singleton = true;
@@ -700,7 +705,7 @@ rbtracer_add(char *query, bool is_slow)
 
       method = idx+1;
 
-    } else if (NULL != (idx = rindex(query, '#'))) {
+    } else if (NULL != (idx = strrchr(query, '#'))) {
       klass_begin = 0;
       klass_end = idx - query;
       is_singleton = false;

server.rb

@@ -1,4 +1,5 @@
 require 'ext/rbtrace'
+require 'tmpdir'
 
 class String
   def multiply_vowels(num)
@@ -26,19 +27,21 @@ def self.run
 
 while true
   proc {
-    Dir.chdir("/tmp") do
-      Dir.pwd
-      Process.pid
-      'hello'.multiply_vowels(3){ :ohai }
-      sleep rand*0.5
+    Dir.mktmpdir do |tmp|
+      Dir.chdir(tmp) do
+        Dir.pwd
+        Process.pid
+        'hello'.multiply_vowels(3){ :ohai }
+        sleep rand*0.5
 
-      ENV['blah']
-      GC.start
+        ENV['blah']
+        GC.start
 
-      reload_test.call
-      Test.run
+        reload_test.call
+        Test.run
 
-      #fib(1024*100)
+        #fib(1024*100)
+      end
     end
   }.call
 end