tcp: Add element socketsrc

tcp: Add element socketsrc

`socketsrc` can be considered a source counterpart to `multisocketsink`.
It can be considered a generalization of `tcpclientsrc` and
`tcpserversrc`:  it contains all the logic required to communicate over
the socket but none of the logic for creating the sockets/establishing
the connection in the first place, allowing the user to accomplish this
externally in whatever manner they wish making it applicable to other
types of sockets besides TCP.

This commit essentially copies the implementation directly from
tcpserversrc.  Later patches will tidy the implementation up and
re-implement `tcpclientsrc` and `tcpserversrc` in terms of `socketsrc`.

See https://bugzilla.gnome.org/show_bug.cgi?id=739546