Fix a bug in r272297 which prevented dumpdev from setting.
!u is not equivalent to (u != 0).
UPD and UDPLite require a checksum. So check for it.

MFC after: 3 days
Make autofs(4) use shared lock for lookups, instead of exclusive one.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Merge remote-tracking branch 'freebsd/master' into hardened/current/master
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/ptrace