Add manpath.d entry

[mattcen]

OpenZFS on OSX installs a paths.d entry (ref etc/paths.d/zfs.in) for binaries, but not an /etc/manpaths.d/ pointing to the appropriate man pages. This commit intends to rectify that missing entry.

Motivation and Context

Man pages don't currently appear in the man path by default.

Description

This is a first draft; I don't have a system on which I can test this build; I have just copied the implementation for etc/paths.d and adapted it as best I can for etc/manpaths.d. I figured this was a better approach than opening an Issue even without the testing

How Has This Been Tested?

I have not yet tested this

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.