runtime: fix plan9 monotonic time, crypto randomness
Open /dev/bintime at process start on Plan 9,
marked close-on-exec, hold it open for the duration of the
process, and use it for obtaining time.
The change to using /dev/bintime also sets up for an upcoming
Plan 9 change to add monotonic time to that file. If the monotonic
field is available, then nanotime1 and time.now use that field.
Otherwise they fall back to using Unix nanoseconds as "monotonic",
as they always have.
Before this CL, monotonic time went backward any time
aux/timesync decided to adjust the system's time-of-day backward.
Also use /dev/random for randomness (once at startup).
Before this CL, there was no real randomness in the runtime
on Plan 9 (the crypto/rand package still had some). Now there will be.