Generate an installation script

This commit adds a `-g` flag to rcup(1) to generate a standalone shell
script. This shell script can then be run again, even on different
computers, to recreate the symlinks.

This allows people to recreate the "download my dotfiles and run
./install.sh" instructions, but with generated code that they do not
need to maintain.

This provides us more freedom with lsrc(1): since rcm can be used to
generate a universal shell script, lsrc(1) now can be harder to install
-- it can depend on a compiler, for example -- because you only need to
install it on one machine.

The generated script is rather limited; this can be improved in future
commits, as desired.