Switch to check for effective user id in r349320, and disable dumping
into existing files for sugid processes.

Despite using real user id pronounces the intent, it actually breaks
suid coredumps, while not making any difference for non-sugid
processes. The reason for the breakage is that non-existent core file
is created with the effective uid (unless weird hacks like SUIDDIR are
configured).

Then, if user enabled kern.sugid_coredump, core dumping should not
overwrite core files owned by effective uid, but we cannot pretend to
use real uid for dumping.

PR: 68905
admbugs: 358
Sponsored by: The FreeBSD Foundation
MFC after: 1 week