Cleanup xlog logging code to be safe and usable for all

This patch reworks the xlog logging code to avoid rebuilding the message into a
fixed size buffer. It also adds two new logging functions xlog_warn and
xlog_err which are replacements for idmap_warn and idmap_err. There use to be
two different variates of these functions with the only difference being that
one flavor tacked on the error string to the end of the message. This
responsibility has been pushed to the called of the function since it
needlessly complicated the function and required us to rebuild the message
strings.

Signed-off-by: David P. Quigley <dpquigl@tycho.nsa.gov>
Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>