Sync our regex code with upstream changes since last time we did this, which

was Tcl 8.4.8.  The main changes are to remove the never-fully-implemented
code for multi-character collating elements, and to const-ify some stuff a
bit more fully.  In combination with the recent security patch, this commit
brings us into line with Tcl 8.5.0.

Note that I didn't make any effort to duplicate a lot of cosmetic changes
that they made to bring their copy into line with their own style
guidelines, such as adding braces around single-line IF bodies.  Most of
those we either had done already (such as ANSI-fication of function headers)
or there is no point because pgindent would undo the change anyway.