[PATCH] unified frontend support for pg_malloc et al and palloc/pfree mulation (was xlogreader-v4)

Hi,

As promised here's a patch to provide palloc emulation for frontend-ish
environments.

The patch:
- makes palloc() into a real function so CurrentMemoryContext doesn't need to be provided
- provides common pg_(malloc,malloc0, realloc, strdup, free) wrappers and removes various versions of those across
differentutilities
 
- removes ugly palloc redefinery for frontend use of backend code (dirmod.c)

Controversial/Unclear things:
- palloc[0] are currently copies of the MemoryContextAlloc[Zero] functions to preclude performance regressions, imo the
levelof duplication is ok though
 
- the common memory management is implemented in [pg]port/palloc.[ch], I am not too happy with the name and location
- pgport/palloc.c is only built in the backend, not sure if there is a nicer way to do this from a make POV
- the different versions of pg_malloc et al used different error signaling methods, I've settled on    fprintf(stderr,
_("outof memory\n"));    exit(EXIT_FAILURE);
 

Results in a nice net removal of code:37 files changed, 218 insertions(+), 621 deletions(-)