uci: introduce confirm method to cancel pending rollbacks without side effects

uci: introduce confirm method to cancel pending rollbacks without side effects

Calling commit with rollback=false to cancel a pending rollback timer can
potentially trigger another commit if no rollback is actually pending.

This introduces a new method "confirm" which cancels the rollback timer
without any other possible side effects.

While being at it, change the return code when trying to commit during
a pending rollback or when trying to rollback without a pending timeout to
UBUS_STATUS_NO_DATA. This is needed to be able to differentiate between a
denied request and an inappropriate state.

Also change the error return code for glob() failures to the more
appropriate UBUS_STATUS_NOT_FOUND.

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