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.