regcomp.c: Make sure /di nodes begining in 's' are EXACTF

regcomp.c: Make sure /di nodes begining in 's' are EXACTF

This is defensive coding.  The previous commit changed things so under
/di a node ending in [Ss] doesn't get made an EXACTFU.  This commit does
the same for nodes that begin with [Ss].  This isn't actually necessary
as one needs two EXACTFU nodes in a row for the problem to occur, and
the previous commit appears to remove the possibility for the first node
being an EXACTFU.  But I'm leery of relying on this.  So this commit
makes sure that a node beginning with 'S' or 's' under /di remains
EXACTF