runq: More macros; Better and more consistent naming

Most existing macros have ambiguous names regarding which index they
operate on (queue, word, bit?), so have been renamed to improve clarity.
Use the 'RQSW_' prefix for all macros related to status words, and
change the status word type name accordingly.

Rename RQB_FFS() to RQSW_BSF() to remove confusion about the return
value (ffs*() return bit indices starting at 1, or 0 if the input is 0,
whereas BSF on x86 returns 0-based indices, which is what the current
code assumes). While here, add a check (under INVARIANTS) that
RQSW_BSF() isn't called with 0 as an argument.

Also, rename 'rqb_bits_t' to the more concise 'rqsw_t', 'struct rqbits'
to 'struct rq_status', its 'rqb_bits' field to 'rq_sw' (it designates an
array of words, not bits), and the type 'rqhead' to 'rq_queue'

Add macros computing a queue index from a status word index and a bit in
order to factorize code. If the precise index of the bit is known,
callers can use RQSW_TO_QUEUE_IDX() to get the corresponding queue
index, whereas if they want the one corresponding to the first
(least-significant): set bit in a given status word (corresponding to
the non-empty queue with lower index in the status word), they can use
RQSW_FIRST_QUEUE_IDX() instead.

Add RQSW_BIT_IDX(), which computes the correspond bit's index in the
corresponding status word. This allows more code factorization (even if
most uses will be eliminated in a later commit) and makes what is
computed clearer.

Reviewed by: kib
MFC after: 1 month
Event: Kitchener-Waterloo Hackathon 202506
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45387