allow empty DOCKER_DOMAIN_SUFFIX

[rooterkyberian]

This patch allows complete removal of DOCKER_DOMAIN_SUFFIX during configuration phase, like this:
./configure --prefix=/usr --with-docker-domain-suffix=""

So instead of ping test.docker you can do just ping test, just like you would inside of the container.

[sitano]

for performance considerations its worth of having some sort of filter for names not to be checked when the suffix is not set. i.e. having dots in the middle (like in google.com) to be excluded. this filter, for example, could be configurable too. I just exclude dots in my branch.

[moschlar]

See also #11

[azazar]

for performance considerations its worth of having some sort of filter for names not to be checked when the suffix is not set. i.e. having dots in the middle (like in google.com) to be excluded. this filter, for example, could be configurable too. I just exclude dots in my branch.

Docker allows dots in container names so dots should be allowed by resolver too. That filter should be optional and disabled by default.