Allow j.sh to be invoked directly in which case package command is passed as first parameter.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
jsh/runtime.j: support for linking into a bin directory, list resolved packages, configuring the bin directory

With these changes:

'jsh runtime get bin' reports the directory to which the runtime bin directory is linked.
'jsh runtime set bin' will configure the link to the runtime bin directory
'jsh runtime link bin' will create one link for every resolved package.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
README.md: updated bootstrap instructions

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>