SUBJECTTHREADSEXCRXP fix

Hi,

My patch sets SUBJECTTHREADSEXCRXP's default value to "(?!)", a regular
expression that matches nothing.

It turns out, unfortunately, that evaluating this regular expression is
very slow.

A better choice: In mhinit.pl, initialize $SubThreadsExcRxp to "". Then, at
mhthread.pl line 228, change

                 unless defined($FirstSub2Index{$tmp}) ||
                        ($tmp =~ /$SubThreadsExcRxp/io) ||
                        (defined($Refs{$index}) &&
                         grep($MsgId{$_}, split(/$X/o, $Refs{$index})));

to

                 unless defined($FirstSub2Index{$tmp}) ||
                        ($SubThreadsExcRxp ne '' &&
                         $tmp =~ /$SubThreadsExcRxp/io) ||
                        (defined($Refs{$index}) &&
                         grep($MsgId{$_}, split(/$X/o, $Refs{$index})));

Eddie