pathmap or dir command on drugs

[Jim Blandy]

I agree with Fernando that the symmetry between pathmap entries and
breakpoints isn't compelling.  The command set should be designed to
fit the application.

It's worth taking care to keep the most common action --- adding a
pathmap --- terse.  Note that the commands for editing the source
directory list are `directory' and `show directories' --- it breaks
the `set/show' symmetry (there's a `show', but no `set' command), but
it's more convenient because it's less verbose.  I think it would be
fine for the commands to be `pathmap' and `show pathmap'.