bridge: Only do base64 decoding in one place in CockpitChannel

There's no need to do this in two places.
bridge: Add a prepare vfunc to CockpitChannel

The queueing of cockpit_channel_ready() and cockpit_channel_close()
was too magical and causing too much complexity.
bridge: When we receive clean close before ready, don't close yet

When we received data or a clean close before the channel is ready
don't close prematurely. Make sure all the data is drained, and
then do the clean close.
bridge: CockpitChannel "disconnected" when transport closes

The CockpitChannel closes unsuccessfully when the transport
closes. This lets implementations react immediately, even if the
transport closes without error.

Closes #1518
bridge: More robust parsing of channel options

The parsing of channel options was quite haphazard, and didn't
detect bad types for values, etc. This is more verbose, but it's more
failure proof and produces better diagnostic messages.
bridge: Better diagnostics for file read/write failures

In the file read/replace channels, put some context in the messages
so we can have a clue as to what's causing it. Previously we had:

cockpit-bridge-Message: /tmp/.V3DRPX/foo: Bad file descriptor
bridge, ws: An empty "problem" in a "close" message is a problem

A missing "problem" is a success. If the "problem" is present no matter
what it's content, then that denotes a problem.
bridge, base ws: Add the "eof" control message and change "close"

The new "eof" control message indicates the end of data in a channel
in a particular direction. It is relevant together with specific
payloads and affects their behavior. It may only be sent once per
direction, and after that point no more messages may be sent in
that direction.

The "eof" provides handshake interlocking for the success case for
payloads where this is necessary.

There is also no longer a "close" handshake. Either end can "close"
a channel, and the channel is considered closed immediately. In the
case of races, further messages are ignored.
common: Fix uninitialized memory access in CockpitTransport

Closes #1531