all: Fix os.Signal chan buffering

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.

Closes #590

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Merge pull request #624 from flynn/fix-signal

all: Fix os.Signal chan buffering