Productize 'path' command

[jcburley]

Add explicit --path= option to specify the path, so an empty path can be handled within $(...) without having to use eval.

Maybe also support --pathvar= to name a var that path itself expands?

Create and install man page.

Have both --help and man page explain possible need to first test for path itself being in $PATH, or don't assign results of $(path ...) (or equivalent) to the $PATH or similar variable directly, instead to a temp var that is then checked for the empty string (which would mean path itself didn't run, i.e. wasn't found, in most cases), assigning to the ultimate variable only if non-empty.