As per discussions on -current, re-add /var/mail (removed in r257842) but

this time with atime=on in support of various software that requires it.

Discussed on:	-current
MFC after:	3 days

usr.sbin/bsdinstall/scripts/zfsboot

@@ -132,6 +132,7 @@ f_isset ZFSBOOT_DATASETS || ZFSBOOT_DATASETS="
 	/var		mountpoint=/var
 	/var/crash	compression=lz4,exec=off,setuid=off
 	/var/log	compression=lz4,exec=off,setuid=off
+	/var/mail	compression=lz4,atime=on
 	/var/tmp	compression=lz4,exec=on,setuid=off
 " # END-QUOTE