Return scoped_fd from open_source_file and find_and_open_source

This changes open_source_file and find_and_open_source to return
scoped_fd, then updates the callers as appropriate, including using
scoped_fd::to_file.

Tested by the buildbot.

gdb/ChangeLog
2018-11-09  Tom Tromey  <tom@tromey.com>

	* common/scoped_fd.h (class scoped_fd): Add move constructor and
	move assignment operator.
	* psymtab.c (psymtab_to_fullname): Update.
	* source.h (open_source_file): Return scoped_fd.
	(find_and_open_source): Likewise.
	* source.c (open_source_file): Return scoped_fd.
	(get_filename_and_charpos): Update.
	(print_source_lines_base): Update.  Use scoped_fd::to_file.
	(forward_search_command): Likewise.
	(reverse_search_command): Likewise.
	(find_and_open_source): Return scoped_fd.
	* tui/tui-source.c (tui_set_source_content): Update.  Use
	gdb_file_up.