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>