swim: move sockaddr_in checkers to swim_proto.h

There are several places where it is necessary to check if a
sockaddr_in is nullified, and to compare a couple of addresses.
Some of them are in swim_proto.c, and more are coming in indirect
SWIM messages patch. The patch moves the checkers into
swim_proto.h so as to be usable from anywhere in SWIM.

Also minor renames are made alongside. 'sockaddr_in' is too long
to use in each related function's name, and is replaced with
'inaddr' by analogue with the standard library.

Part of #3234