Using the %m printf format more

Hi hackers,

I just noticed that commit d93627bc added a bunch of pg_fatal() calls
using %s and strerror(errno), which could be written more concisely as
%m.  I'm assuming this was done because the surrounding code also uses
this pattern, and hadn't been changed to use %m when support for that
was added to snprintf.c to avoid backporting hazards.  However, that
support was in v12, which is now the oldest still-supported back branch,
so we can safely make that change.

The attached patch does so everywhere appropriate. One place where it's
not appropriate is the TAP-emitting functions in pg_regress, since those
call fprintf() and other potentially errno-modifying functions before
evaluating the format string.

- ilmari