2.3.1: build fixes

This release is just a build fixing release.
No need to upgrade for users of 2.3.0.

Includes a static binary build against libsodium 1.0.8, libev 4.22, libpcap 1.7.4 on debian etch

Static package can be used on any i386 system:

tar -C / -xpzf mlvpn_static_ev_4.22_libsodium_1.0.10
# for debian:
insserv mlvpn