xmalloc => pg_malloc

While looking around to fix the pg_malloc(0) issue, I noticed that
various other pieces of code such as pg_basebackup have essentially
identical functions, except they're called xmalloc().  I propose to
standardize all these things on this set of names:
pg_mallocpg_malloc0    (for malloc-and-zero behavior)pg_calloc    (randomly different API for
pg_malloc0)pg_reallocpg_freepg_strdup

Any objections?
        regards, tom lane