Unfortunately it seems trickier to support --config globally
The non-trivial changes are in notmuch.c; most of the other changes
consists of blindly inserting two lines into every subcommand.
Apart from the status codes for format mismatches, the non-zero exit
status codes have been arbitrary. Make the cli consistently return
either EXIT_SUCCESS or EXIT_FAILURE.
The extra path component added by the lib is a magic value that the
caller just has to know. This is demonstrated by the current code,
which indeed has "xapian.old" both sides of the interface. Use the
backup path provided by the lib caller verbatim, without adding
anything to it.
