Revert "unix/dgram: return EAGAIN instead of ENOBUFS when O_NONBLOCK set"

This API change led to unexpected consequences with Go runtime. The
Go runtime emulates blocking sockets over non-blocking sockets and
for that uses available event dispatcher on the target OS, which is
kevent(2) if availabe, with OS independent layer on top. It expects
that if whatever O_NONBLOCK socket returned ever EAGAIN, then it is
supposed to be reported as writable by the event dispatcher. kevent(2)
would never report a unix/dgram socket, since they never change their
state, they always are writeable. The expectations of Go are not
literally specified by SUS, however they are in its spirit. The SUS
specifies EAGAIN for send(2) as "The socket's file descriptor is marked
O_NONBLOCK and the requested operation would block" [1]. This doesn't
apply to FreeBSD unix/dgram socket, it never blocks on send(2).

Thus, changing API trying to mimic Linux was a mistake. But what about
the problem we tried to fix? Discussed that with Max Dounin of nginx,
and we agreed that the log bomb described shall be fixed on nginx side,
and it actually isn't specific to FreeBSD, may happen with nginx on any
non-Linux system with a certain configuration.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/send.html

This reverts commit 65572cade35a93add2168a7a012f808ac499218b.