multipath: add option to change the number of error messages

multipath: add option to change the number of error messages

This patch adds a new default config parameter, log_checker_err.  It accepts
two values, "once" and "always", and defaults of "always".  It controls
how multipathd logs checker error messages.  If it's set to "once", only the
first checker error message is logged at logging level 2. All future messages
are logged at level 3, until the device is restored or removed. If it's set
to "always", all messages are logged at level 2, like multipathd currently does.

This version actually compiles.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>