Refactor fileprefix() and filecopy() to use warn() instead of err()

Refactor fileprefix() and filecopy() to use warn() instead of err()
to display error message, and to return error indications (NULL and
-1 respectively).  Use the error indications in write_efisystem()
to unwind in the face of more error conditions. In other cases just
exit(1) to emulation current behaviour.

ok deraadt@