mime: use Scanner to read mime files during init Also close the file when we're done.
mime: use Scanner to read mime files during init
Also close the file when we're done.

R=bradfitz
CC=golang-dev
https://codereview.appspot.com/7363045

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