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

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.