Link to the Web manpages

To give people a preview of the joy they will expect to find when they
start using rcm, link to the HTML manpages from the README.
Allow the user to override SYMLINK_DIRS in lsrc(1)

This adds a `-s` that can be used to override the `SYMLINK_DIRS` config,
or the `-S` flag, to lsrc(1).
A test for mkrc -s

The -s flag already worked in mkrc, so add a test for it.
Attempt to fix mkrc(1) -s

mkrc(1)'s `-s` flag depends on rcup(1) accepting the `-s` flag. Pass it
through, similarly to `-S`.

However, rcup(1) does not yet accept `-s`, so this breaks everything.
rcup(1) takes -s
Passing mkrc(1) tests

Default `force_symlink` to a sentinal that we don't check for.

Fix the test: it moves the directory, then symlinks the files.
rcdn(1) supports -s
Documentation for lsrc(1), rcdn(1), rcup(1) -s