imports: minor fixes

These are mostly comments for CL 24941 that didn't
get sent quickly enough.

strings.Trim(`\"`) works, but by accident.
It trims all leading and trailing "s and \s,
but there are never leading or trailing \s.

Semantic line breaks and punctuation cleanup.

The reflow of comments in the pkgName == "main" if
block is to silence this spurious vet failure:

fix.go:247: +build comment must appear before package clause and be followed by a blank line

Plain sync.Once values are enough.

Change-Id: I241f3025031b6f21605da78ea52066713a203327
Reviewed-on: https://go-review.googlesource.com/24983
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>