Support rc files without leading dots

This adds the `-U` option to lsrc(1), rcup(1), and rcdn(1) commands; its
argument is an exclusion pattern. Any file matching this pattern is
symlinked without the leading dot.

There is also a `-u` option to undo a `-U`. The `UNDOTTED` setting in
rcrc(5) can be used to set it permanently.

The mkrc(1) command has `-U` and `-u` flags. They take no argument.