[Nmh-workers] locking default (post-1.5)

[David Levine]

I just commited a changed on master (NOT on the 1.5 branch)
to default the locking type to:

  AIX, Cygwin, and Linux: fcntl
  FreeBSD: flock

dot locking is still the default on other platforms.  The
--with-locking configure option to override the default is
also unchanged.

I did this because some Cygwin installations would behave
badly with dot locking.  I believe that the other defaults
above make sense.  If not, or if others should be added,
please let me know.

David