syscall: lazily populate the environment cache on Plan 9
syscall: lazily populate the environment cache on Plan 9

This decreases the amount of system calls during the
first call to Getenv. Calling Environ will still read
in all environment variables and populate the cache.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/6939048

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang