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.)

src/bin/pg_dump/compress_io.c

@@ -138,7 +138,7 @@ AllocateCompressor(int compression, WriteFunc writeF)
 		exit_horribly(modulename, "not built with zlib support\n");
 #endif
 
-	cs = (CompressorState *) pg_calloc(1, sizeof(CompressorState));
+	cs = (CompressorState *) pg_malloc0(sizeof(CompressorState));
 	cs->writeF = writeF;
 	cs->comprAlg = alg;