branches: 1.2.4;
lint: warn about constant assignment in operand to '&&' or '||'

Seen in bin/sh/redir.c:1084 for errno, which is fine.

Seen in usr.sbin/mopd/mopprobe/mopprobe.c:235, which looks suspicious
but hasn't been touched since 1997.

lint: add warning for '!(var = constant)'

This catches expressions like 'ATF_CHECK(errno = ENOENT)', which was
intended to use '==' instead of '='.