Remove cleanups from solib-target.c

Remove cleanups from solib-target.c

This removes the remaining cleanups from solib-target.c, plus a VEC,
by using std::vector and std::unique_ptr.

Tested by the buildbot.

gdb/ChangeLog
2019-01-17  Tom Tromey  <tom@tromey.com>

	* solib-target.c (lm_info_target_p): Remove typedef.  Don't
	declare VEC.
	(solib_target_parse_libraries): Change return type.
	(library_list_start_segment, library_list_start_section)
	(library_list_end_library, library_list_start_library); Update.
	(solib_target_free_library_list): Remove.
	(solib_target_parse_libraries): Remove cleanup.  Change return
	type.
	(solib_target_current_sos): Update.