Add some mention of the functions used by protocol implementations,
upcalls, and other general routines.
Not all functionality is documented; in particular:
o the *at() variants, which are useful only for implementing the
corresponding syscalls.
o soconnect2(), also only used to implement a syscall (socketpair()).
o sockargs(), which is essentually unused and only tangentially
socket-related.
o selsocket(), which is commented as being present solely for use by
netncp and netsmb.
o getsockaddr(), which is just a convenience shortcut for copyin().
Reviewed by: jhb (previous version)
Approved by: hrs (mentor)
