periodic: Make daily diff(1) output as small is possible

Make, by default, daily diff(1) ignore whitespace changes and the unified output
a context of zero (0) lines. This reduces output of unrelated lines in e-mails
delivered to root.

PR:		270266
Approved by:	jrm (mentor), karels
MFC after:	1 month
Relnotes:	yes
Differential Revision:	https://reviews.freebsd.org/D42762

(cherry picked from commit e2f6bafc38)

share/man/man5/periodic.conf.5

@@ -650,6 +650,9 @@ All scripts must be absolute path names.
 Set to the arguments to pass to the
 .Xr diff 1
 utility when generating differences.
+The default is
+.Fl b
+.Fl U Cm 0 .
 .El
 .Pp
 The following variables are used by the standard scripts that reside in

usr.sbin/periodic/etc/daily/200.backup-passwd

@@ -40,7 +40,7 @@ case "$daily_backup_passwd_enable" in
 	    then
 		[ $rc -lt 1 ] && rc=1
 		echo "$host passwd diffs:"
-		diff ${daily_diff_flags:--u} -I '^#' $bak/master.passwd.bak /etc/master.passwd |\
+		diff ${daily_diff_flags} -I '^#' $bak/master.passwd.bak /etc/master.passwd |\
 			sed 's/^\([-+ ][^:]*\):[^:]*:/\1:(password):/'
 		mv $bak/master.passwd.bak $bak/master.passwd.bak2
 		cp -p /etc/master.passwd $bak/master.passwd.bak || rc=3
@@ -57,7 +57,7 @@ case "$daily_backup_passwd_enable" in
 	    then
 		[ $rc -lt 1 ] && rc=1
 		echo "$host group diffs:"
-		diff ${daily_diff_flags:--u} $bak/group.bak /etc/group
+		diff ${daily_diff_flags} $bak/group.bak /etc/group
 		mv $bak/group.bak $bak/group.bak2
 		cp -p /etc/group $bak/group.bak || rc=3
 	    fi

usr.sbin/periodic/etc/daily/210.backup-aliases

@@ -34,7 +34,7 @@ case "$daily_backup_aliases_enable" in
 	    then
 		[ $rc -lt 1 ] && rc=1
 		echo "$host aliases diffs:"
-		diff ${daily_diff_flags:--u} $bak/aliases.bak /etc/mail/aliases
+		diff ${daily_diff_flags} $bak/aliases.bak /etc/mail/aliases
 		mv $bak/aliases.bak $bak/aliases.bak2
 		cp -p /etc/mail/aliases $bak/aliases.bak || rc=3
 	    fi

usr.sbin/periodic/periodic.conf

@@ -31,7 +31,7 @@ anticongestion_sleeptime=3600
 # that output.  $daily_output might be set to /var/log/daily.log if you
 # wish to log the daily output and have the files rotated by newsyslog(8)
 #
-daily_diff_flags=""					# flags for diff output
+daily_diff_flags="-b -U 0"				# flags for diff output
 daily_output="root"					# user or /file
 daily_show_success="YES"				# scripts returning 0
 daily_show_info="YES"					# scripts returning 1