Launch the shell, passing it the path to the mk-osreldate script, rather
than launching the script directly and relying on #! to launch the shell.
This avoids problems when the source is mounted with the noexec flag.

MFC this together with r255775.

Approved by:	re (kib)
MFC after:	2 weeks
Increase the chance of the buffer write from the bufdaemon helper
context to succeed.  If the locked vnode which owns the buffer to be
written is shared locked, try the non-blocking upgrade of the lock to
exclusive.

PR:	kern/178997
Reported and tested by:	Klaus Weber <fbsd-bugs-2013-1@unix-admin.de>
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (marius)
Pre-acquire the filedesc sx when a possibility exists that the later
code could need to remove a kqueue from the filedesc list.  Global
lock is already locked, which causes sleepable after non-sleepable
lock acquisition.

Reported and tested by:	pho
Reviewed by:	jmg
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Approved by:	re (gjb)