Allow attaching to user-specified domain

[m-ronnblom]

In case the application does not have control of the domains file, but has the required information (server IPs, X.509 certs etc), it may set the PAF_DOMAINS variable and write its own domains file.

This is somewhat cumbersome and, in the case the application runs in a environment where it may not set its environment variable (e.g., Java), it may not even be possible.

In light of this, it could be useful to have a paf_attach() variant that takes a domain name, all the information normally found in a domain entry in the domains file as input.