MFC r278479,278494,278525,278545,278592,279237,280410:

MFC r278479,278494,278525,278545,278592,279237,280410:

This change merges devctl notification for userland coredumps.

r278479 (rpaulo):

  Notify devd(8) when a process crashed.

  This change implements a notification (via devctl) to userland when
  the kernel produces coredumps after a process has crashed.
  devd can then run a specific command to produce a human readable crash
  report.  The command is most usually a helper that runs gdb/lldb
  commands on the file/coredump pair.  It's possible to use this
  functionality for implementing automatic generation of crash reports.

  devd(8) will be notified of the full path of the binary that crashed and
  the full path of the coredump file.

r278494 (rpaulo):

  Sanitise the coredump file names sent to devd.

  While there, add a sysctl to turn this feature off as requested by kib@.

r278525 (rpaulo):

  Remove a printf and an strlen() from the coredump code.

r278545 (rpaulo):

  Restore the data array in coredump(), but use a different style to
  calculate the length.

r278592 (rpaulo):

  Remove check against NULL after M_WAITOK.

r279237 (kib):

  Keep a reference on the coredump vnode for vn_fullpath() call.  Do it
  by moving vn_close() after the point where notification is sent.

r280410 (rpaulo):

  Disable coredump_devctl because it could lead to leaking paths to jails.

Approved by:	re

git-svn-id: svn://svn.freebsd.org/base/stable/10@322848 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f