Increase footprint of %m and reduce strerror()

Hi all,

Since commit d6c55de1, we support %m in the in-core port for printf
and such.  And it seems to me that we could do better for the frontend
code by reducing the dependency to strerror().

One advantage of doing a switch, or at least reduce the use of
strerror(), would be to ease the work of translators with more error
messages unified between the frontend and the backend.  A possible
drawback is that this could be a cause of minor conflicts when
back-patching.  Always easy enough to fix, still that can be
annoying.

Thoughts?
--
Michael