ss: Fix for added diag support check

ss: Fix for added diag support check

Commit 9f66764e308e9 ("libnetlink: Add test for error code returned from
netlink reply") changed rtnl_dump_filter_l() to return an error in case
NLMSG_DONE would contain one, even if it was ENOENT.

This in turn breaks ss when it tries to dump DCCP sockets on a system
without support for it: The function tcp_show(), which is shared between
TCP and DCCP, will start parsing /proc since inet_show_netlink() returns
an error - yet it parses /proc/net/tcp which doesn't make sense for DCCP
sockets at all.

On my system, a call to 'ss' without further arguments prints the list
of connected TCP sockets twice.

Fix this by introducing a dedicated function dccp_show() which does not
have a fallback to /proc, just like sctp_show(). And since tcp_show()
is no longer "multi-purpose", drop it's socktype parameter.

Fixes: 9f66764e308e9 ("libnetlink: Add test for error code returned from netlink reply")
Signed-off-by: Phil Sutter <phil@nwl.cc>