notmuch-mutt: Fix tagging issues

notmuch-mutt: Fix tagging issues

This patch fixes three issues with "notmuch-mutt tag":

1. The message_id was not shell quoted.
   Thanks to Jason Miller for the bug report and patch.

2. The tags passed into tag_action() were not being properly quoted.
   The "join before shell_quote" was combining multiple tags into a
   single argument to notmuch tag: '+one -two -three' instead of
   '+one' '-two' '-three'.

3. A "--" was added between the tags and search-term as shown in the
   current notmuch-tag man page.

Thanks to Tomi Ollila for suggesting the simple fix of using
the list form of system(), which bypasses the shell.