ecpg: move some functions into a new file ecpg/preproc/util.c.

ecpg: move some functions into a new file ecpg/preproc/util.c.

mm_alloc and mm_strdup were in type.c, which seems a completely
random choice.  No doubt the original author thought two small
functions didn't deserve their own file.  But I'm about to add
some more memory-management stuff beside them, so let's put them
in a less surprising place.  This seems like a better home for
mmerror, mmfatal, and the cat_str/make_str family, too.

Discussion: https://postgr.es/m/2011420.1713493114@sss.pgh.pa.us