Moved the `memoize` decorator to `salt.utils.decorators`.

The internal decorator wrapper is now decorated with `functools.wraps`. This solves a missing documentation issue shown on CentOS 5 under python 2.6 for a salt module function which had a docstring.
`salt.modules.freebsdpkg` is now using `salt.utils.decorators.memoize`
`salt.modules.freebsdservice` is now using `salt.utils.decorators.memoize`
`salt.modules.grub_legacy` is now using `salt.utils.decorators.memoize`
`salt.modules.launchctl` is now using `salt.utils.decorators.memoize`
`salt.modules.nginx` is now using `salt.utils.decorators.memoize`
`salt.modules.pkgin` is now using `salt.utils.decorators.memoize`
`salt.modules.selinux` is now using `salt.utils.decorators.memoize`
`salt.modules.smartos_imgadm` is now using `salt.utils.decorators.memoize`
`salt.modules.smartos_vmadm` is now using `salt.utils.decorators.memoize`
`salt.modules.zfs` is now using `salt.utils.decorators.memoize`
`salt.modules.zpool` is now using `salt.utils.decorators.memoize`
Hide _namespaced_function from salt's loader.

This is needed because in CentOS 5, under python 2.6 that function is flagged as not having a CLI example from the test suite.
Show what the salt function returned in case of assertion errors.
Merge pull request #6688 from s0undt3ch/develop

Remove trailing white-space.
Merge pull request #6686 from s0undt3ch/hotfix/hide-namespaced_function

Hide _namespaced_function from salt's loader.
Merge pull request #6687 from s0undt3ch/hotfix/show-additional-info

Show what the salt function returned in case of assertion errors.
Merge pull request #6685 from s0undt3ch/hotfix/memoize-decorator

move and fix memoize decorator
Fixed `integration.states.pip.PipStateTest.test_pip_installed_errors` error regex.

It was failing to match on FreeBSD.