The obj library's flag functions are (mostly) light wrappers
around the standard library flag package. Use the flag package
directly where possible.

Most uses of the 'count'-type flags (except for -v) only check
against 0, so they can safely be replaced by bools. Only -v
and the flagfns haven't been replaced.

Debug has been turned into a slice of bools rather than ints.
There was a copy of the -v verbosity in ctxt.Debugvlog, so don't use
Debug['v'] and just use ctxt.Debugvlog.

Updates #16818

Change-Id: Icf6473a4823c9d35513bbd0c34ea02d5676d782a
Reviewed-on: https://go-review.googlesource.com/27471
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>