Publish config key capabilities.

Publish config key capabilities.

Every driver now publishes its device option config keys, i.e. the
list fetched with sr_config_list(SR_CONF_DEVICE_OPTIONS), with a
set of flags indicating which methods are implemented by the driver
for that key.

The config keys are OR'ed with any combination of SR_CONF_GET,
SR_CONF_SET and SR_CONF_LIST. These are defined as the high bits
of the uint32_t config key. Clients can OR config keys with
SR_CONF_MASK to strip out these bits. This mask will be kept up to
date if other bits are added to the capabilities list; clients MUST
therefore use SR_CONF_MASK for this.

Some keys don't have capability bits added, such as the informative
device type keys (SR_CONF_MULTIMETER, SR_CONF_OSCILLOSCOPE, ...) and
SR_CONF_CONTINUOUS.

Scan options do not have capabilities bits.