Re-work level setting on checks code

Currently each of the semantic checks in checks.c has a "level" between
IGNORE and ERROR.  This single level makes it awkward to implement the
semantics we want for toggling the checks on the command line.

This patch reworks the code to instead have separate boolean flags for
warning and error.  At present having both flags set will have the same
effect as having just the error flag set, but this can change in the
future.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>