MEDIUM: tools: support specifying explicit address families in str2sa_range()

MEDIUM: tools: support specifying explicit address families in str2sa_range()

This change allows one to force the address family in any address parsed
by str2sa_range() by specifying it as a prefix followed by '@' then the
address. Currently supported address prefixes are 'ipv4@', 'ipv6@', 'unix@'.
This also helps forcing resolving for host names (when getaddrinfo is used),
and force the family of the empty address (eg: 'ipv4@' = 0.0.0.0 while
'ipv6@' = ::).

The main benefits is that unix sockets can now get a local name without
being forced to begin with a slash. This is useful during development as
it is no longer necessary to have stats socket sent to /tmp.