Introduce the gmCapable flag for use with 802.1AS clocks.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Inhibit sync messages from unwilling 802.1AS ports.

According to 802.1AS, ports are always expected to transmit announce
messages, even if they never want to become the grand master. Instead
of using a slave only BMC state machine as in 1588, 802.1AS offers a
"grand master capable" flag which allows clocks to not send sync
messages.

This patch keeps a port from transmitting sync (but not announce)
messages when there is no other master.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Add a configuration file option for the 802.1AS gmCapable flag.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>