Make find_separate_debug_file* return std::string

Make find_separate_debug_file* return std::string

This patch makes the find_separate_debug_file* functions return
std::string, which allows to get rid of some manual memory management
and one cleanup.

gdb/ChangeLog:

	* build-id.c (find_separate_debug_file_by_buildid): Return
	std::string.
	* build-id.h (find_separate_debug_file_by_buildid): Return
	std::string.
	* coffread.c (coff_symfile_read): Adjust to std::string.
	* elfread.c (elf_symfile_read): Adjust to std::string.
	* symfile.c (separate_debug_file_exists): Change parameter to
	std::string.
	(find_separate_debug_file): Return std::string.
	(find_separate_debug_file_by_debuglink): Return std::string.
	* symfile.h (find_separate_debug_file_by_debuglink): Return
	std::string.