map: make tunnel encapsulation limit support configurable (FS#1501)

map: make tunnel encapsulation limit support configurable (FS#1501)

Be compatible with ISPs which don't support the destination option header containing
the tunnel encapsulation limit as reported in FS#1501.
Setting the uci parameter encaplimit to ignore; allows to disable the insertion
of the destination option header in the map-e packets.
Otherwise the tunnel encapsulation limit value can be set to a value from 0 till 255
by setting the encaplimit uci parameter accordingly.
If no encaplimit value is specified the default value is 4 as before.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>