[3769] DHPCv4 now uses PID file, made pid test common

[3769] DHPCv4 now uses PID file, made pid test common

src/lib/testutils/dhcp_test_lib.sh.in
    - server_pid_file_test() - common test for any server
    to verify PID file management

src/bin/d2/tests/d2_process_tests.sh.in
    remmoved duplicate_server_start_test
    now calls server_pid_file_test

Added PID file creation to DHCP4
    src/bin/dhcp4/dhcp4_messages.mes
    -  added log DHCP4_ALREADY_RUNNING

    src/bin/dhcp4/main.cc
    - added logic to create the PID and catch
    exception specific to PID conflict

    src/bin/dhcp4/tests/Makefile.am
    - exports KEA_PIDFILE_DIR

    src/bin/dhcp4/tests/dhcp4_process_tests.sh.in
    - added call to server_pid_file_test

    src/bin/dhcp4/tests/dhcp4_unittests.cc
    - main(int argc, char* argv[])
    sets env var KEA_PIDFILE_DIR