darwin: fix unnecessary include headers

This file doesn't use any Cocoa functions, CoreFoundation.h is enough here.
This line causes compilation error on iOS environment.
darwin: rename darwin-getproctitle.m

Rename it to darwin-getproctitle.c, it doesn't need an Objective-C
compiler. Fix up -Wpedantic warnings about void to function pointer
casts and include <ApplicationServices/ApplicationServices.h> to get
the GetCurrentProcess() function prototype.