Rest of the MMDF-style mail drop support removal.

Rest of the MMDF-style mail drop support removal.
Only Unix-style (mbox) mail drops are supported now. Their location
is in `mailspool', which is reported in the  configure summary now.
If users like to have different locations, they can use the environment
variable MAILDROP or the profile entry MailDrop to overrule the default.
From mts.conf, mmdfldir and mmdflfil were removed.
msgchk(1) received several cleanups.
I touched sbr/m_getfld.c ... Wooohh ...