mail/mailman: fix empty group/user name regression in 2.1.39_2

...and drop files/patch-configure-in, which is no longer needed;
neither is autoreconf needed to be reinstated.

We can cheat the test -z "$VAR" inside the MM_FIND_{GROUP,USER}...
configure[.in] macros by putting the proper contents into the
proper environment variables.

To fix the regression in...
PR: 274991
Reported by: leres@