[3769] DHCPv6 now uses PID file

[3769] DHCPv6 now uses PID file

Added PID file creation to DHCP6
    src/bin/dhcp6/dhcp6_messages.mes
    -  added log DHCP6_ALREADY_RUNNING

    src/bin/dhcp6/main.cc
    - added logic to create the PID and catch
    exception specific to PID conflict

    src/bin/dhcp6/tests/Makefile.am
    - exports KEA_PIDFILE_DIR

    src/bin/dhcp6/tests/dhcp6_process_tests.sh.in
    - added call to server_pid_file_test

    src/bin/dhcp6/tests/dhcp6_unittests.cc
    - main(int argc, char* argv[])
    sets env var KEA_PIDFILE_DIR