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

Unbreak sparc64 after r276630 by calling __sparc_sigtramp_setup signal
trampoline as part of the MD __sys_sigaction again.

Submitted by: kib (initial versions)
MFC after: 3 days

Install the userland signal trampoline when sigaction is first called,
instead of on startup. This fixes binary compatibility of dynamically
linked binaries from before the signal code move.

Suggested by: wollman (a long time ago)