server: Support loading as a shared library (#459)

On non-rooted Android in some contexts one can't execute binaries from
various locations but can load them using dlopen() and other tricks.

If a user manages to trigger dlopen() + dlsym() they can invoke main()
directly without further hacks.

server/frida-server.version

@@ -3,6 +3,8 @@
     # FreeBSD needs these two:
     __progname;
     environ;
+    # Allow users to load the server as a shared library with dlopen() and call main() directly
+    main;
 
   local:
     *;