protosw: refactor protosw and domain static declaration and load

[glebius]

Summary

o Assert that every protosw has pr_attach. Now this structure is
only for socket protocols declarations and nothing else.
o Merge struct pr_usrreqs into struct protosw. This was suggested
in 1996 by wollman@ (see 7b187005d18ef), and later reiterated
in 2006 by rwatson@ (see 6fbb9cf860dcd).
o Make struct domain hold a variable sized array of protosw pointers.
For most protocols these pointers are initialized statically.
Those domains that may have loadable protocols have spacers. IPv4
and IPv6 have 8 spacers each (andre@ dff3237ee54ea).
o For inetsw and inet6sw leave a comment noting that many protosw
entries very likely are dead code.
o Refactor pf_proto_[un]register() into protow_[un]register().
o Isolate pr_*_notsupp() methods into uipc_domain.c

[glebius]

Bugfix + rebase

[glebius]

bugfix

[melifaro]

sys/kern/uipc_domain.c

383

Is this needed?

404

Is this special case needed? E.g. if adding domains with AF_UNSPEC is not a valid use case, we would naturally return NULL after search.

473

maybe name prp like proto_slot so it can be a bit more readable?

497

Shouldn't we catch it on attach instead?

[glebius]

Committed e7d02be19d40063783d6b8f1ff2bc4c7170fd434