Use cpp_strerror() wherever possible, and use autoconf for portability

strerror_r is not portable; on Gnu libc it returns char * and sometimes
does not fill in the supplied buffer.  Use autoconf to test which
version this platform uses and adapt.

Clean up the random calls to strerror and strerror_r (along with all
their private little one-use buffers) and regularize the code to use
cpp_strerror almost everywhere.  Where changed, any negation of the
error code is also removed, since cpp_strerror() will do that.

Note: some tools were using their own calls to strerror/strerror_r, so
will now get a (%d) in their output that wasn't there before; hence
the change to test/cli/monmaptool/print-nonexistent.t

Fixes: #8041
Signed-off-by: Dan Mick <dan.mick@inktank.com>