Introduce a hooks layer for the MPSAFE TTY layer.

One of the features that prevented us from fixing some of the TTY
consumers to work once again, was an interface that allowed consumers to
do the following:

- `Sniff' incoming data, which is used by the snp(4) driver.

- Take direct control of the input and output paths of a TTY, which is
  used by ng_tty(4), ppp(4), sl(4), etc.

There's no practical advantage in committing a hooks layer without
having any consumers. In P4 there is a preliminary port of snp(4) and
thompsa@ is busy porting ng_tty(4) to this interface. I already want to
have it in the tree, because this may stimulate others to work on the
remaining modules.

Discussed with:	thompsa
Obtained from:	//depot/projects/mpsafetty/...

1 files changed, 1 insertions, 0 deletions

diff --git a/usr.sbin/pstat/pstat.c b/usr.sbin/pstat/pstat.c
index 51828cb..a1e81b3 100644
--- a/usr.sbin/pstat/pstat.c
+++ b/usr.sbin/pstat/pstat.c
@@ -312,6 +312,7 @@ static struct {
 	{ TF_EXCLUDE,	'X' },
 	{ TF_BYPASS,	'l' },
 	{ TF_ZOMBIE,	'Z' },
+	{ TF_HOOK,	's' },
 
 	{ 0,	       '\0' },
 };