enabling -Werror=assign-enum for kernel

  Hi,

modern clang has a nice feature: it allows to catch enum misuse.  For example:

enum foo { A = 1, B } var;
var = 1;	/* success */
var = A;	/* success */
var = B;	/* success */
var = 3;	/* fail */

What about enums that are used as flags? For that it has
__attribute__((flag_enum)).  This will allow using combination of values and
clearing them:

var = A | B;	/* success */
var &= ~A;	/* success */
var = 3;	/* fail */

This seems like a nice enforcement of a good code.  I already found a few
hamrless bugs with it and one in e1000 that could be a real bug.  It also
highlights some sketchy code that better be refactored.

I have a branch where LINT is compilable with -Werror=assign-enum:

https://github.com/freebsd/freebsd-src/compare/main...glebius:FreeBSD:Werror%3Dassign-enum

There are basically three parts that require work:

1) The most common violators are SYSINIT(9)s that use (SI_SUB_FOO + 1) as
argument.  I have posted a pack of reviews to cover that.

2) hwpmc has enum pmc_event that is generated by preprocessor and its use is
sketchy,    thus files that utilize HWPMC_HOOKS are temporarily excluded from
the enforcements.

3) I was too tired to look at OFED, thus just disabled it in ${OFED_C} in
kern.mk

Comments are welcome!

-- 
Gleb Smirnoff