Don't ask questions if we are not running interactively.

When creating invitations or using them to join a VPN, and the tinc command is
not run interactively (ie, when stdin and stdout are not connected or
redirected to/from a file), don't ask questions. If normally tinc would ask for
a confirmation, just assume the default answer instead. If tinc really needs
some input, just print an error message instead.

In case an invitation is used for a VPN which uses a netname that is already in
use on the local host, tinc will store the configuration in a temporary
directory. Normally it asks for an alternative netname and then renames the
temporary directory, but when not run interactively, it now just prints the
location of the unchanged temporary directory.
Document Weight and also allow it to be set from tinc.conf.