SUBJECTTHREADSEXCRXP fix

[Eddie Kohler <kohler@aciri.org>]

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