isisd: Add SRv6 locator name to SRv6 configuration

isisd: Add SRv6 locator name to SRv6 configuration

Add the name of the SRv6 locator to use with IS-IS to the per-area SRv6
configuration. If an SRv6 locator is not configured for an IS-IS
instance, the locator name is an empty string. When an IS-IS instance is
configured to use an SRv6 locator,  the locator name stores the name of
the selected locator.

Subsequent commits will add the possibility to set and unset an SRv6
locator for a specific IS-IS instance.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>