Add the documentation for the `behavior usid` command to zebra.
When the `behavior usid` command is set, a flag is added to the locator
to indicate that the locator is a uSID locator. When a locator is
specified as a uSID locator, the bgpd will install SRv6 behaviors with
the uSID in the dataplane and use the SRv6 uSID codepoints in the BGP
update message.