Add a savecore_enable variable support for the savecore rc script

By default set to 'YES' so it does not change the current behaviour for users,
this variable allows to decide to not extract crach dumps from the dump
device at boot time by setting it to "NO" in rc.conf.

Sponsored by:	Gandi.net

etc/defaults/rc.conf

@@ -603,6 +603,7 @@ chkprintcap_enable="NO"	# Run chkprintcap(8) before running lpd.
 chkprintcap_flags="-d"	# Create missing directories by default.
 dumpdev="AUTO"		# Device to crashdump to (device name, AUTO, or NO).
 dumpdir="/var/crash"	# Directory where crash dumps are to be stored
+savecore_enable="YES"	# Extract core from dump devices if any
 savecore_flags="-m 10"	# Used if dumpdev is enabled above, and present.
 			# By default, only the 10 most recent kernel dumps
 			# are saved.

etc/rc.d/savecore

@@ -10,6 +10,7 @@
 . /etc/rc.subr
 
 name="savecore"
+rcvar="savecore_enable"
 desc="Save a core dump of the operating system"
 start_cmd="savecore_start"
 start_precmd="savecore_prestart"

share/man/man5/rc.conf.5

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd February 28, 2016
+.Dd April 29, 2016
 .Dt RC.CONF 5
 .Os
 .Sh NAME
@@ -3511,6 +3511,12 @@ to not run
 at boot time when
 .Va dumpdir
 is set.
+.It Va savecore_enable
+.Pq Vt bool
+If set to
+.Dq Li NO ,
+disable automatic extraction of the crash dump from the
+.Va dumpdev .
 .It Va savecore_flags
 .Pq Vt str
 If crash dumps are enabled, these are the flags to pass to the