projtool.pl: allow existing projects to be specified by path
Where an existing project is required, accept a path to that
project and make a best effort attempt to translate that path
to a project name before giving up with an error.
For safety, make the remove command continue to only accept
project names. The prune command, by its very nature, only
accepts project names as it's looking for missing directories.
Add a note about all this to the beginning of the help output
including a mention of the convenient "." shortcut.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>