fnmatch(3): improve POSIX conformance.
In a recent interpretation[1], "\\" shall return a non-zero value
(indicating either no match or an error).
The fix involves a change over r254091 and now the behavior matches the
Sun/IBM/HP closed source implementations and also likely musl libc.
Submitted by: Joerg Schilling <joerg at schily.net>
MFC after: 1 week