Add serv specific ip_pool and l4_redirect_ip_pool

[cygnusb]

This PR allows to use a l4-redirect-ip-pool Option on interface specific configurations:

interface=enp8s0.500,mode=L2,shared=1,start=dhcpv4,ifcfg=1,ipv6=1,ip-pool=vlan500,l4-redirect-ip-pool=noauth-vlan500

If not specified, global l4-redirect pools are used.

In addition this patch changes the following additional things:

• remove ipv6 and dpv6 pools on l4-redirect (is this really intended/useful to have ip6 connectivity if l4-redirect is used ?!?)
• add additional log output, which ip pool is used (might be too verbosive?!?)
• unify two loglines into one on start of l4-redirect sessions

I am not fully sure if these additional changes are all ok. Especially the first one. Please comment if needed.

[cygnusb]

Moved to accel-ppp/accel-ppp repo