Make npth_{s,g}etname_np portable.

These functions are usually used for debugging. On systems where
they are not easy to support they will now simply return ENOSYS.

• configure.ac: Check for pthread_setname_np and pthread_getname_np.
• src/libnpth.vers: Add npth_setname_np and npth_getname_np.
• src/npth.c (npth_setname_np, npth_getname_np): New.
• src/npth.h (npth_setname_np, npth_getname_np): Turn macros into

functions.