luci-mod-admin-full: restructure and fix backup, restore and sysuprade (#517)

luci-mod-admin-full: restructure and fix backup, restore and sysuprade (#517)

Do not use standard post security checking for actions that require file upload
since reading the token value will trigger parsing of the http message body
before the file upload handler has been set, which causes LuCI to buffer the
entire request body in memory.

In order to simplify the code and logic flow, split action_flashops() into
separate handlers for reset, backup, restore and sysupgrade.

Let the backup restore and sysupgrade handlers use the new test_post_security()
method in luci.dispatcher to perform token checking *after* setting the upload
handler.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>