crashinfo: Create core.txt.last symlink

When saving a coredump, savecore(8) maintains .last symlinks for the
info and vmcore artifacts, but not for the crashinfo text report.

Make crashinfo(8) create the link, pointing at the current
core.txt.<bounds> file.

This makes /var/crash/core.txt.last track the same core dump as
info.last and vmcore.last.

[mhorne: I tweaked the submission, such that the link will be created as
soon as the core.txt.X file is generated; not only after a successful
report has been written.]

Signed-off-by:	Ricardo Branco <rbranco@suse.de>
Reviewed by:	mhorne
MFC after:	1 week
Pull Request:	https://github.com/freebsd/freebsd-src/pull/2199

usr.sbin/crashinfo/crashinfo.sh

@@ -149,11 +149,13 @@ fi
 VMCORE=$CRASHDIR/vmcore.$DUMPNR
 INFO=$CRASHDIR/info.$DUMPNR
 FILE=$CRASHDIR/core.txt.$DUMPNR
+LINK=$CRASHDIR/core.txt.last
 HOSTNAME=`hostname`
 
 if $BATCH; then
 	echo "Writing crash summary to $FILE."
 	exec > $FILE 2>&1
+	ln -sf $FILE $LINK
 fi
 
 GDB=/usr/local/bin/gdb
@@ -204,6 +206,7 @@ machine=$(gdb_command $KERNEL 'printf "%s", machine')
 if ! $BATCH; then
 	echo "Writing crash summary to $FILE."
 	exec > $FILE 2>&1
+	ln -sf $FILE $LINK
 fi
 
 echo "$HOSTNAME dumped core - see $VMCORE"