mount: support -i; pass through -f and -n to helpers as necessary

function                                             old     new   delta
mount_it_now                                         298     345     +47
singlemount                                          776     772      -4

Signed-off-by: Colin Watson <cjwatson@ubuntu.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

include/usage.h

@@ -2900,6 +2900,9 @@
 	IF_NOT_FEATURE_MTAB_SUPPORT( \
      "\n	-f		Dry run" \
 	) \
+	) \
+	IF_FEATURE_MOUNT_HELPERS( \
+     "\n	-i		Don't run mount helper" \
 	) \
 	IF_FEATURE_MTAB_SUPPORT( \
      "\n	-n		Don't update /etc/mtab" \