Return unique_xmalloc_ptr from some solib.c functions

Return unique_xmalloc_ptr from some solib.c functions

This changes a couple of solib.c functions -- exec_file_find and
solib_find -- to return a unique_xmalloc_ptr, and then fixes up the
users.  This allows the removal of some cleanups.

This also changes solib_bfd_open to not take ownership of its
argument.  I think this change is somewhat cleaner.

gdb/ChangeLog
2018-02-14  Tom Tromey  <tom@tromey.com>

	* solist.h (exec_file_find, solib_find): Return
	unique_xmalloc_ptr.
	(solib_bfd_fopen): Take a const char *.
	* solib.c (solib_find_1): Return unique_xmalloc_ptr.
	(exec_file_find, solib_find): Likewise.
	(solib_bfd_fopen): Do not take ownership of "pathname".
	(solib_bfd_open): Use unique_xmalloc_ptr.
	* solib-darwin.c (darwin_bfd_open): Use unique_xmalloc_ptr.
	* solib-aix.c (solib_aix_bfd_open): Use unique_xmalloc_ptr.
	* infrun.c (follow_exec): Use unique_xmalloc_ptr.
	* exec.c (exec_file_locate_attach): Use unique_xmalloc_ptr.