bozo: Use buffered I/O to send notifier data

bozo: Use buffered I/O to send notifier data

Currently, popen() is used to open a pipe to the notifier program, then
notifier messages are formatted in memory then sent with unbuffered I/O,
then the pipe is immediately closed.

Instead of allocating buffers and formatting the notifier string, just
use the buffered stream.  Check for write errors with ferror(), and do
not ignore the pclose() return code.

Change-Id: Ib2bd692591171b4f380fc25bec56d4db898d883e
Reviewed-on: https://gerrit.openafs.org/14818
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>