connection: store entire kernel_flags bitfield in connection

drop bool starter from connection, and add two more bits to the flags,
signalling whether the connection want do accept FDs and mmap()ed areas.
Also add a test for FDs in kdbus_conn_enqueue_kmsg().
store received/installed file descriptors in pre-composed data record
add userspace header includes if kdbus.h is used from userspace
fix payload vec interation
Makefile: move tests to test/

This is needed in order to refinde implicit rules for building binaries
for the target, not the build host. IOW: we can now cross-compile the
test binaries.
test/Makefile: use -std=gnu99

anonymous unions are not conform to the C99 standard.