libctf: drop error-prone ctf_strerror

libctf: drop error-prone ctf_strerror

This utility function is almost useless (all it does is casts the result
of a strerror) but has a seriously confusing name.  Over and over again
I have accidentally called it instead of ctf_errmsg, and hidden a
time-bomb for myself in a hard-to-test error-handling path: since
ctf_strerror is just a strerror wrapper, it cannot handle CTF errnos,
unlike ctf_errmsg.  It's astonishingly lucky that none of these errors
have crept into any commits to date.

Fuse it into ctf_errmsg and drop it.

libctf/
	* ctf-impl.h (ctf_strerror): Delete.
	* ctf-subr.c (ctf_strerror): Likewise.
	* ctf-error.c (ctf_errmsg): Stop using ctf_strerror: just use
	strerror directly.