Unix domain sockets incorrectly listed as IPv6.
Merge pull request #2010 from yossigo/unixdomain-info-string

Unix domain sockets incorrectly listed as IPv6.
anetPeerToString() refactoring and more explicit checks.

Related to PR #2010.
anetPeerToString(): make unix sockets distinguishable.

Following the CLIENT LIST output format, we prefix the unix socket
address with a "/" so that it is different than an IPv4/6 address.
This makes parsing simpler.

Related to #2010.