libc/signal and include/signal.h: Rename sigset() to signal(). sigset() is...

libc/signal and include/signal.h: Rename sigset() to signal().  sigset() is the System V function; signal() is the obsoleted POSIX name.  These seem to be equivalent.  Neither are relevant in modern POSIX programming but the name signal() probably has better credentials.  sigset() is now relegated to a #define in signal.h that makes it equivalent to signal().