Copy stable/11@r303970 to releng/11.0 as part of the 11.0-RELEASE
cycle.

Prune svn:mergeinfo from the new branch, and rename it to RC1.

Update __FreeBSD_version.

Use the quarterly branch for the default FreeBSD.conf pkg(8) repo and
the dvd1.iso packages population.

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

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

Fix warnings and bump WARNS to 6
- Garbage collect argc/argv (-Wunused)
- sleep(3) will always return an unsigned int; don't check for return codes <0
(-Wsign-compare)

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

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.