Add pthread_getname_np and pthread_setname_np for Hurd

We use thread_get_name and thread_set_name to get and set the thread
name, so nothing is stored in the thread structure since these functions
are supposed to be called sparingly.

One notable difference with Linux is that the thread name is up to 32
chars, whereas Linux's is 16.

Also added a mach_RPC_CHECK to check for the existing of gnumach RPCs.