Add pppd sleep UCI option

Workaround for #107, improvement of !167 (merged)

A user should be allowed to tune or switch off introduced "sleep" during pppd connection setup. This patch introduce such UCI option pppd_sleep defaulting to 10 seconds, the original hardcoded value.

Numeric value of 0 or less will turn the sleep completely off and every non-numeric value will cause it to fall to a default value (10).

Also fix hack patches numbering