Make relocate_{path,gdb_directory} return std::string

Make relocate_{path,gdb_directory} return std::string

This simplifies memory management. I've also changed some global variables
to std::string accordingly (which store the result of these functions),
but not all because some are used with add_setshow_optional_filename_cmd
which requires a char*.

gdb/ChangeLog:

2019-09-11  Christian Biesinger  <cbiesinger@google.com>

	* auto-load.c (auto_load_expand_dir_vars): Update.
	* defs.h (gdb_datadir): Change to std::string.
	(python_libdir): Likewise.
	(relocate_gdb_directory): Change return type to std::string.
	* guile/guile.c (gdbscm_data_directory): Update.
	(initialize_scheme_side): Update.
	* jit.c (jit_reader_dir): Change to std::string.
	(jit_reader_load_command): Update.
	* main.c (gdb_datadir): Change to std::string.
	(python_libdir): Likewise.
	(set_gdb_data_directory): Update.
	(relocate_path): Change to return std::string.
	(relocate_gdb_directory): Change to return std::string.
	(relocate_gdbinit_path_maybe_in_datadir): Update.
	(captured_main_1): Update.
	* python/python.c (do_start_initialization): Update.
	* top.c (show_gdb_datadir): Update.
	* xml-syscall.c (xml_init_syscalls_info): Update.
	(init_syscalls_info): Update.