Use router_get_my_routerinfo() rather than desc_routerinfo

The following functions use desc_routerinfo where they could use router_get_my_routerinfo():

• check_descriptor_ipaddress_changed
• router_compare_to_my_exit_policy
• router_my_exit_policy_is_reject_star
• router_get_my_descriptor
• check_descriptor_bandwidth_changed

This could cause issues if desc_routerinfo needs to be rebuilt, because some of these functions don't call router_get_my_routerinfo() first.

Others have comments about desc_routerinfo that might need to be updated:

• dirserv_get_routerdescs
• router_compare_to_my_exit_policy
  • Also change "IPv6 and IPv6 entries" to "IPv4 and IPv6 entries"