Rename USE_GNU_REGEX to USE_SYS_REGEX, since it has nothing to do with
GNU.  This doesn't affect the param users/builders would pass to cmake
or the resulting code, so it's just internal cleanup.
Move the USEM4 vars over into twm.h.
Move captivename over into twm.h, and take out an extern that was
pulling it in elsewhere.
Eliminate includes of ctwm.h in ctwm files.
This file is now cleared of all but the client-lib stuff, so rename it
and its uses.
Add idempotency guards.
Mechanical rename of twm.h to ctwm.h now that the name is cleared.
Shuffle header file names around.

Previously we had a ctwm.h that was mostly related to the libctwm
example-client stuff, except it also had a few vars that had snuck in
over the years relating to ctwm stuff.  With those moved to our more
normal header, that ctwm.h can be renamed to ctwm_client.h.  And then
with that done, we can move twm.h over to ctwm.h, since it's been
20-some years since we were twm and even the .c file has long since
been moved.

This is mostly a mechanical change.