odhcp6c: make ds-lite/map tunnel encapsulation limit support configurable (FS#1501)

odhcp6c: make ds-lite/map 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 for dynamic created ds-lite/map
interfaces.
Setting the uci parameter encaplimit_dslite/map to ignore; allows to disable the insertion
of the destination option header for the dynamic created ds-lite/map interface.
Otherwise the tunnel encapsulation limit value can be set to a value from 0 till 255
by setting the encaplimit_dslite/map uci parameter accordingly.

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