Add updated bitcoin.conf from wiki + regtest

In this bitcoin.conf I use basically the [config from wiki](https://en.bitcoin.it/wiki/Running_Bitcoin) plus a few changes:
- removed reference to allow payment via IP
- added reference to regtest

It's important that eventually we get this default config for Debian up to standards e.g. all options in the default config, commented with the default value they have when running. This is a first step.
Instead of `*` syntax, we've switched to subnet-based matching recently so

    rpcallowip=10.1.1.34/255.255.255.0
    rpcallowip=1.2.3.4/24
    rpcallowip=2001:db8:85a3:0:0:8a2e:370:7334/96
    ...