Doug's portability fix of my setgid inc autodetection had a caching bug -- if

Doug's portability fix of my setgid inc autodetection had a caching bug -- if
you re-ran configure, uip/Makefile would be corrupted, and installation would
fail on OSes where inc needs to be setgid.

Implemented a new kind of email address masquerading.  When a user writes a
custom "From:" header in a draft, nmh uses it rather than generating one.
However, the user's true address is used in the SMTP envelope "From:" and is
revealed in the "Sender:" header.  Now, when mmailid is set to non-zero, the
envelope "From:" uses the address specified in the draft "From:" header, and
there is no "Sender:" header.  This is useful when sending on behalf of a remote
POP3 account or when remote mail robots incorrectly use the envelope "From:" in
preference to the body "From:".