Remove cleanups from solib.c

Remove cleanups from solib.c

This removes a couple of cleanups from solib.c, replacing one with
std::string and another with unique_xmalloc_ptr.

2018-02-08  Tom Tromey  <tom@tromey.com>

	* solib.c (solib_find_1): Use std::string.
	(solib_bfd_fopen): Use unique_xmalloc_ptr.