[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 different utilities
- 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 level of
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, _("out of memory\n"));
exit(EXIT_FAILURE);

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