notmuch-mutt: replace shell pipeline with internal pipe processing

notmuch-mutt: replace shell pipeline with internal pipe processing

The shell pipeline used to symlink files based in search results
to "cache" directory for mutt(1) to use was prone to portability
problems (due to /bin/sh differences).

The replacement executes `notmuch search` without intermediate shell
(so shell_quote was removed in this case), reads the filenames from
piped output and symlinks files internally.