PR3858
* print.c: Add -DRELAY_HOST=<hostname> and -DRELAY_GUEST=<hostname>
options support.
(_stp_print_flush): Disable irqs if -DRELAY_* option is specified.
* print_old.c: Export stp_print_flush to other modules if the
-DRELAY_HOST option is specified.
* print_new.c: Ditto.
* transport.c (_stp_transport_init): Reduce relay buffer size to
64KB*2 if -DRELAY_GUEST is specified.
* systemtap.printf/sharedbuf.exp: New test for buffer sharing option.
* systemtap.printf/sharedbuf.stp: Ditto.
* systemtap.printf/hello.stp: Ditto.
* systemtap.printf/hello2.stp: Ditto.