vio::uart is now based on vio::file instead of vio::io.
Updated syscall checklist in configure.in since vio::file
and vio::uds are using additional calls.
Changed vio::uds interface by disabling vio::file::open methods
in favour of vio::uds::join and vio::uds::host since argument
'reuse' was removed by calling unlink in vio::uds::~uds, which
was also used to distinguish vio::uds::open methods between
either joining or hosting. Thus we now have to store sockaddr_un
as a member to pass the file path to unlink.