kamailio-5.x: update to 5.1.6

kamailio-5.x: update to 5.1.6

Minor version bump. Two CVE patches can be removed as they're already
included in the source. One patch was refreshed.

Also:

  - https://sources.openwrt.org is added as primary source URL to lessen
    the load on kamailio upstream
  - Build/Configure is defined as empty (because there is no configure
    script in the source tree
  - patch is added to fix dp_replace(); the function was first added in the
    5.1 release and didn't work; patch was accepted upstream

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>