Add support to configure strongswan via uci.

uci support is based on the following sections
-ipsec : Global config items belonging in the strongswan.conf file
-remote : Defines the remote peer(s)
-tunnel : Defines the IPSec connections in tunnel mode
-transport : Defines the IPSec connections in transport mode
-crypto_proposal : Defines the different crypto proposals

Signed-off-by: Pierre Lebleu <pme.lebleu@gmail.com>
Signed-off-by: Gino Peeters <peeters.gino@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>

Signed-off-by: Steven Barth <steven@midlink.org>