--lock-flags now configurable, and some mu_argp reorg

[Sam Roberts]

I made some changes, folks can tell me what they think:

- mailutils allows you to write non-GPL programs, those programs
  should be able to use mu_argp_parse() -> "licence" is now
  its own capability

- not all programs use the mailspool, but those that do will want
  to know how to lock it -> added a "mailbox" capability with
  --mail-spool and --lock-flags in it, maybe it should be more
  granular still.

- I couldn't get control over the f**ing order argp was printing
  options for --help. I wanted:

 <utility specific>

 <auth>

 <daemon>

  (and other major groups with their own headings)

 Common options:

 lock flags, mailspool, log flags, etc., things without their own
 headings


 --licence
 <builtin: --help, --version, --usage>


There was some code to auto-number things, but it didn't work, and
I couldn't figure out why, so I hard coded groups, and now it works.

If theres a nicer way to get a nice order, I'd love to hear.


- just because there are other mail and from and sieve utilities in
  the world, doesn't mean that I should have to rename the config
  files for mine as ~/.mu.???rc, IMHO. It is handy to isolate/group
  rcfiles sometimes, though, so now if ~/.mailutils is a directory,
  then that directory is searched for the rc files, instead of ~/.

  I can add back the .mu. prefix for config files if thats not
  sufficient, but I really like the directory of config files, an
  idea I shamelessly stole from mutt.

Sam