Make delim_string_to_char_ptr_vec return an std::vector

Make delim_string_to_char_ptr_vec return an std::vector

This patch makes delim_string_to_char_ptr_vec and all related functions
use std::vector of gdb::unique_xmalloc_ptr.  This allows getting rid of
make_cleanup_free_char_ptr_vec.  Returning a vector of
unique_xmalloc_ptr instead of std::string allows to minimize the impacts
on the calling code.  We can evaluate later whether we could/should
return a vector of std::strings instead.

gdb/ChangeLog:

	* common/gdb_vecs.h (make_cleanup_free_char_ptr_vec): Remove.
	(delim_string_to_char_ptr_vec): Return std::vector of
	gdb::unique_xmalloc_ptr.
	(dirnames_to_char_ptr_vec_append): Take std::vector of
	gdb::unique_xmalloc_ptr.
	(dirnames_to_char_ptr_vec): Return std::vector of
	gdb::unique_xmalloc_ptr.
	* common/gdb_vecs.c (delim_string_to_char_ptr_vec_append):
	Take std::vector of gdb::unique_xmalloc_ptr, adjust the code.
	(delim_string_to_char_ptr_vec): Return an std::vector of
	gdb::unique_xmalloc_ptr, adjust the code.
	(dirnames_to_char_ptr_vec_append): Take an std::vector of
	gdb::unique_xmalloc_ptr, adjust the code.
	(dirnames_to_char_ptr_vec): Return an std::vector of
	gdb::unique_xmalloc_ptr, adjust the code.
	* auto-load.c (auto_load_safe_path_vec): Change type to
	std::vector of gdb::unique_xmalloc_ptr.
	(auto_load_expand_dir_vars): Return an std::vector of
	gdb::unique_xmalloc_ptr, adjust the code.
	(auto_load_safe_path_vec_update): Adjust.
	(filename_is_in_auto_load_safe_path_vec): Adjust.
	(auto_load_objfile_script_1): Adjust.
	* build-id.c (build_id_to_debug_bfd): Adjust.
	* linux-thread-db.c (thread_db_load_search): Adjust.
	* source.c (add_path): Adjust.
	(openp): Adjust.
	* symfile.c (find_separate_debug_file): Adjust.
	* utils.c (do_free_char_ptr_vec): Remove.
	(make_cleanup_free_char_ptr_vec): Remove.

gdb/gdbserver/ChangeLog:

	* server.c (parse_debug_format_options): Adjust to
	delim_string_to_char_ptr_vec changes.
	* thread-db.c (thread_db_load_search): Adjust to
	dirnames_to_char_ptr_vec changes.