Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Use '%zd' format specifier for ssize_t

Found by: clang

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.