libblacklist: Do not use %m for logging, use strerror(errno)

The blacklist library can accept a function to use for logging,
defaulting to vsyslog(), if no function is specified. Make the
blacklist library use strerror(errno) explicitly, instead of %m,
so that the passed in function does not need to support the
syslog specific placeholder.

This matches a change already submitted and accepted upstream.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation