MFC r303151:
Provide counter_warning(9) KPI.

MFC r303155:
Hide counted_warning(9) under #ifdef _KERNEL braces.

MFC r298337:
r298337 (by cem):

pty(4): Use strlcpy to guarantee destination buffer isn't overrun

The devtoname() name is strcpyed into a small stack buffer. Sure, we always
expect the name to be ttyXX (or ptyXX). If that's the case, strlcpy() doesn't
hurt.

CID: 1006768

MFC r293825:
Switch legacy pty clone handler to use make_dev_s(9).

MFC r294594:
Remove printf only useful for debugging.

Approved by: re (marius)

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix whitespace inconsistencies in the TTY layer and its drivers owned by me.

Finish r210923, 210926. Mark some devices as eternal.

MFC after: 2 weeks

Move ptmx into pty(4).

Now that pty(4) is a loadable kernel module, I'd better move /dev/ptmx
in there as well. This means that pty(4) now provides almost all
pseudo-terminal compatibility code. This means it's very easy to test
whether applications use the proper library interfaces when allocating
pseudo-terminals (namely posix_openpt and openpty).

Allow pty(4) to be loaded as a kld.

Unfortunately, the wrappers that are present in pts(4) don't have the
mechanics to allow pty(4) to be unloaded safely, so I'm forcing this kld
to return EBUSY. This also means we have to enable some extra code in
pts(4) unconditionally.

Proposed by: rwatson