uipc: rework recvfrom, getsockname, getpeername

Stop using <foo>_args structs as part of internal kernel APIs. Add
a kern_recvfrom and adjust getsockname and getpeername's equivalent
functions to take individual arguments rather than a uap pointer.

Adopt a convention from CheriBSD that a function interacting with
userspace pointers and sitting between the sys_<foo> syscall and
kern_<foo> implementation is named user_<foo>.

Reviewed by: kib, imp