Implement basic support for (kernel) rt signals.

include/bits/signal.h

redox-rt/src/arch/x86_64.rs

redox-rt/src/signal.rs

redox-rt/src/sys.rs

src/header/signal/linux.rs

src/header/signal/mod.rs

src/platform/linux/signal.rs

src/platform/redox/signal.rs

tests/Makefile

tests/sigqueue.c