openbsd_pkg: handle pkg_add quirks better.

This fixes a problem when trying to install a package with a specific version
number from a local directory and the local directory is checked after a remote
repository:

Error from http://ftp.eu.openbsd.org/pub/OpenBSD/[...]/packagename-1.0.tgz
ftp: Error retrieving file: 404 Not Found
packagename-1.0: ok
openbsd_pkg: Handle another pkg_add gotcha

* Add '-m' to pkg_add incovation to get access to the "packagename-1.0: ok"
  message.
* Watch for that message if we are about to fail because of stderr in
  package_present().
openbsd_pkg: sync package_latest().

This diff syncs package_latest() with the changes to package_present().
I have not managed to figure out how to handle the cornercases where
stderr is set but the command has not failed, so leave a FIXME blob for
other adventurers.
openbsd_pkg: some whitespace cleanup.
Merge pull request #3493 from eest/openbsd_pkg-fixes

openbsd_pkg corner case fixes