os/signal: No access to the siginfo_t struct

[quentinmit]

The siginfo_t struct that is delivered with signals contains various useful information about the signal. In some cases, this information is necessary to implement the kernel API of various features. For example, fcntl(F_SETLEASE) requires that your process receive a SIGIO when the lock is broken, and the si_fd field in the siginfo_t struct tells you what filehandle the kernel is notifying your process about.

Please provide an interface that will allow Go code to access the information in the siginfo_t struct.

[rsc]

We'd need to figure out a nice, portable API to do this.