/etc/rc.d/pf: Fix for multi-line pf_fallback_rules

Setting multiple pf_fallback_rules in /etc/rc.conf as per the
documentation produces invalid pf syntax due to the lack of echo
quoting $pf_fallback_rules in /etc/rc.d/pf. Adding quotes around
the $pf_fallback_rules echo maintains newlines needed for valid
pfctl syntax. Provided patch resolves the issue

Also updating rc.conf(5) to reflect that multi-line pf_fallback_rules
should not include a trailing backslash (\) as line breaks are
needed when passing rules to pfctl via stdin.

PR:		288197
Reviewed by:	kp
MFC after:	2 weeks

libexec/rc/rc.d/pf

@@ -38,7 +38,7 @@ pf_fallback()
 		$pf_program -f "$pf_fallback_rules_file" $pf_flags
 	else
 		warn "Loading fallback rules: $pf_fallback_rules"
-		echo $pf_fallback_rules | $pf_program -f - $pf_flags
+		echo "$pf_fallback_rules" | $pf_program -f - $pf_flags
 	fi
 }