[Nmh-workers] Making --enable-pop the default (or manditory)

[Ken Hornstein]

In the "reducing the number of knobs and/or #ifdefs" vein ...

Right now POP support for inc is turned off by default.  Thinking
about this, it makes no sense to me ... the amount of code is
relatively small, and we've run into a case recently where we had a
submitted patch that broke POP support badly (I am guessing because
they didn't turn it on).  So at the very least --enable-pop should
be the default.

But then ... I am wondering if it should even be optional?  It would
reduce a number of ifdefs, make the build system simpler, and I'm having
a hard time trying to think of why you wouldn't want it compiled in even
if you never used it.

Thoughts?

--Ken