Support file perms for install_data and install_subdir

With the 'install_mode' kwarg, you can now specify the file and
directory permissions and the owner and the group to be used while
installing. You can pass either:

* A single string specifying just the permissions
* A list of strings with:
  - The first argument a string of permissions
  - The second argument a string specifying the owner or
    an int specifying the uid
  - The third argument a string specifying the group or
    an int specifying the gid

Specifying `false` as any of the arguments skips setting that one.

The format of the permissions kwarg is the same as the symbolic
notation used by ls -l with the first character that specifies 'd',
'-', 'c', etc for the file type omitted since that is always obvious
from the context.

Includes unit tests for the same. Sadly these only run on Linux right
now, but we want them to run on all platforms. We do set the mode in the
integration tests for all platforms but we don't check if they were
actually set correctly.

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