Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Add a regression test to detect if waiting on the I/O serialization lock
on socket buffers is interruptible or not, which detacts the regression I
introduced recently in 7-CURRENT (spotted by alfred). This test passes
in older -CURRENT, and with the as-yet uncommitted sx_xlock_sig and
sblock fix patches.