libsystemd-dhcp: Add capability to print out test steps
libsystemd-dhcp: Factor out test code for DHCP message verification

Add a callback function that will handle the test case specific checking.
libsystemd-dhcp: Add a full DHCP address acquisition test case

Upon startup the DHCP library will immediately send a DHCP Discover.
Handle the additional DHCP Discover checking and DHCP Offer sending
in test_addr_acq_recv_discover(). Write a zero UDP checksum, the
received XID and the faked MAC address into place to create a DHCP
message the library can accept.

Follow exactly the same steps when receiving a DHCP Request in
test_addr_acq_recv_request() and send the Ack. Check that the proper
addresses have been found by the DHCP library before exiting the
main loop and finishing the test.
fix SECCOMP_CFLAGS usage

SECCOMP_CFLAGS must be in the global CFLAGS as <seccomp.h> is
included in core/execute.h. when seccomp.h is not in the standard
path.(i.e openSUSE has it in /usr/include/pkg/libseccomp/, precisely to
catch this kind of bugs) compiling systemd fails.