cfg.mk: use a single regex for all non-reentrant functions

cfg.mk: use a single regex for all non-reentrant functions

The prohibit_nonreentrant syntax-check rule spawns a new shell
for every non-reentrant function we know, to make it easier
to mention the function name in the error message, with the _r
appended.

Since the line with the offending function is already printed
and some of the functions on our list do not have a _r counterpart,
compile them into one big regex and use a more generic error message
to save time.