openbsd_pkg: add upgrade argument.

Running the module with the argument "upgrade=yes" invokes an upgrade of
all installed packages.

While here clean up some comments.

Functionality requested by @qbit.
Refactor openbsd package upgrade change to work more like yum/apt
Simplify logic by using required=True.
openbsd_pkg: revert "changed" simplification.

module.exit_json() does not like when the "changed" variable contains a
match object:

TypeError: <_sre.SRE_Match object at 0x81e2ae58> is not JSON serializable
Merge pull request #3 from eest/openbsd_pkg-upgrade

openbsd_pkg: upgrade all packages when name is '*'