libmultipath/checkers: replace message by msgid

libmultipath/checkers: replace message by msgid

Replace the character array "message" in struct checker with
a "message ID" field.

The generic checker code defines a couple of standard message IDs
and corresponding messages. Checker-specific message IDs start
at CHECKER_FIRST_MSG. Checkers that implement specific message
IDs must provide a table for converting the IDs into actual log
messages.

This simplifies the checker data structure and the handling of
checker messages in the critical checker code path. It comes at
the cost that only constant message strings are supported. It
turns out that only a single checker log message (in the emc_clariion
checker) was dynamically generated, and the missing information can
be provided with a standard condlog message.

Follow-up patches implement this for the existing checkers.

Signed-off-by: Martin Wilck <mwilck@suse.com>