[PATCH] Add use of asprintf()

There is a lot of code around that does something like

char *val = malloc(...obscure computation...);
sprintf(val, "...", ...);

This can be had simpler, with asprintf(). It is not standard, but it's
more widely accepted in C libraries than strlcpy for example. The above
code would simply become

char * val;
asprintf(&val, "...", ...);

and asprintf() will figure out how much memory it needs by itself.

The attached patch should speak for itself.

I have supplied a few variants:

- asprintf() is the standard function, supplied by libpgport if
necessary.

- pg_asprintf() is asprintf() with automatic error handling (like
pg_malloc(), etc.)

- psprintf() is the same idea but with palloc.

I didn't touch most places involving MAXPGPATH. There was a discussion
recently about reducing its usage to save memory. That would work well
on top of my patch.

I did some desultory performance testing, which didn't result in any
concerns. Most of the changes are not in performance-critical paths.

If you're very picky you might notice that the psprintf() implementation
asserts that vsnprintf() does not return negative results. Very old
versions of glibc did that (before 2.1/February 1999), and this issue is
referenced in the current code of copy.c (via be4b8a86). But I think
this issue is too ancient now to be of concern.