branch-merge: clean up socket name

The plymouth client talks to the plymouth daemon via
an abstract socket.  There aren't a lot of "rules" for
the name of the abstract socket, and so plymouth has just
used "/ply-boot-protocol" for the name.  While this is
perfectly valid, a number programs (hal, udev, etc) use
the convention "/org/freedesktop/program".

"/org/freedesktop/plymouthd" is certainly much nicer than
"/ply-boot-protocol" so we're going to change to that, and
fall back to the old name for compatibility.

One other niggle is trailing zeros in the name.  The socket
address is stored in a fixed size buffer.  Traditionally,
programms would pass the size of the entire socket structure,
including the full size of the aaddress buffer to bind and
connect.  This means that any NUL bytes in the address buffer
after the address become part of the address.  This means users
looking at /proc/net/unix will see all the extra NUL bytes.

One trick that some programs employee to skirt around this problem,
is to pass only the size of the structure less the trailing
NUL bytes of the address buffer to bind and connected.  While maybe
not 100% kosher, this works okay in practice.

plymouth will now use that trick as well.