flags: fix handling of log flag
flags: fix handling of log flag

It was missing several things:

1) The Set method didn't set the flag.
2) The String method didn't read the flag.
3) It didn't call log.Set in init to correlate the flag and the default log settings.

Now upspin -? includes this:

  -log level
    	level of logging: debug, info, error, disabled (default info)

Fixes #27.

Change-Id: I30cef66a289db864d504f64abe626dfef1bd7444
Reviewed-on: https://upspin-review.googlesource.com/2671
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin