Force some directories to be symlinks

Typically a directory structure is copied instead of symlinked, while
files are symlinked. However, some cases require symlinked dirs: git
submodules, vim plugins, and so on.

This introduces a `SYMLINK_DIRS` option for rcrc(5) that takes a
space-separated list of "exclude patterns". Any directory matching these
patterns is symlinked.

This also introduces a `-S` argument for lsrc(1), rcup(1), and rcdn(1).
This argument takes a pattern, for one-off directory symlinking. It can
be repeated.

This also introduces `-S` and `-s` for mkrc(1). `-S` will re-install the
files as symlinks, and `-s` will not.

This does work with `-C`, though perhaps unintuitively - we don't know
what the user means in this case. However, it will not crash.

Bug: `-s` does not work right if `SYMLINK_DIRS` is set. Bug #36
addresses this.