bridge: Fix double free in cockpit_channel_ready()

For base64 channels we were freeing the decoded payload twice.

Closes #1515
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
bridge: Allocate enough space for base64 data

Follow the GLib documentation guidelines for allocating enough
space for base64 encoded/decoded data.

Closes #1516
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
tools: Fix make-rpms on recent Fedora 21

There's a lot of default output on Fedora 21 by default, and this
completely breaks make-rpms. Add --quiet to make-rpms to fix this.

Closes #1527
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
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