Standardize naming of malloc/realloc/strdup wrapper functions.

Standardize naming of malloc/realloc/strdup wrapper functions.

We had a number of variants on the theme of "malloc or die", with the
majority named like "pg_malloc", but by no means all.  Standardize on the
names pg_malloc, pg_malloc0, pg_realloc, pg_strdup.  Get rid of pg_calloc
entirely in favor of using pg_malloc0.

This is an essentially cosmetic change, so no back-patch.  (I did find
a couple of places where psql and pg_dump were using plain malloc or
strdup instead of the pg_ versions, but they don't look significant
enough to bother back-patching.)