libiberty, include: add bsearch_r

libiberty, include: add bsearch_r

libctf wants a bsearch that takes a void * arg pointer to avoid a
nonportable use of __thread.

bsearch_r is required, not optional, at this point because as far as I
can see this obvious-sounding function is not implemented by anyone's
libc.  We can easily move it to AC_LIBOBJ later if it proves necessary
to do so.

include/
	* libiberty.h (bsearch_r): New.
libiberty/
	* bsearch_r.c: New file.
	* Makefile.in (CFILES): Add bsearch_r.c.
	(REQUIRED_OFILES): Add bsearch_r.o.
	* functions.texi: Regenerate.