pgsql: Sync our regex code with upstream changes since last time we did

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/backend/regex:
regc_color.c (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/regex/regc_color.c?r1=1.8&r2=1.9)
regc_cvec.c (r1.5 -> r1.6)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/regex/regc_cvec.c?r1=1.5&r2=1.6)
regc_lex.c (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/regex/regc_lex.c?r1=1.7&r2=1.8)
regc_locale.c (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/regex/regc_locale.c?r1=1.8&r2=1.9)
regc_nfa.c (r1.5 -> r1.6)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/regex/regc_nfa.c?r1=1.5&r2=1.6)
regcomp.c (r1.45 -> r1.46)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/regex/regcomp.c?r1=1.45&r2=1.46)
regerror.c (r1.27 -> r1.28)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/regex/regerror.c?r1=1.27&r2=1.28)
pgsql/src/include/regex:
regcustom.h (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/regex/regcustom.h?r1=1.6&r2=1.7)
regguts.h (r1.6 -> r1.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/regex/regguts.h?r1=1.6&r2=1.7)