Notify causes package signal to relay incoming signals to c.
If no signals are listed, all incoming signals will be relayed to c.
Otherwise, just the listed signals will.
Package signal will not block sending to c: the caller must ensure
that c has sufficient buffer space to keep up with the expected
signal rate. For a channel used for notification of just one signal value,
a buffer of size 1 is sufficient.
This package is not yet implemented on Plan 9 and Windows.