use strongswan instead of openswan and add options for certificates, mru and mtu

[lkundrak]

This was migrated over from my fork, since this one is the master one and I'm deleting mine. Submitted by @frenetic1

Hi,
I created a patch for strongswan which also adds support for certificates and setting mru and mtu. I didn't adapt the gui for those options because I don't use gnome.

[lkundrak]

Hi. Thanks for the patch.

However, the proper upstream for this is here:
https://github.com/seriyps/NetworkManager-l2tp/

Please submit your branch there.

Also, some distributions such as RHEL use Libreswan. I guess that one is close to the Openswan which was originally supported here; would it be possible to update the patchset in a way that uses either libreswan (assume it's the same as openswan for now) or strongswan depending on which binary can it find? Sort of what NetworkManager-openswan does for choosing between openswan and libreswan? (I would be able to test and fix the libreswan part if you implement it).

Thank you.

Also:

I created a patch for strongswan which also adds support for certificates and setting mru and mtu. I didn't adapt the gui for those options because I don't use gnome.

Please split those into separate patches so that it's easier for review.

[frenetic1]

I had already forgotten the sleep thingy. This one is fixed now. Can you test libreswan? It should be doing the same as it did in the version without strongswan.

[lkundrak]

Thank you. Applied & pushed your patches.

Didn't really test with Libreswan but I guess it was not very well tested before either.