Simplify program name in $?USAGE if in PATH

Before:
```
$ panda
Usage:
  /Users/tsmith/local/src/p6/rakudo/install/languages/perl6/site/bin/panda [--notests] [--nodeps] install [<modules> ...] -- Install the specified modules
  /Users/tsmith/local/src/p6/rakudo/install/languages/perl6/site/bin/panda [--installed] [--verbose] list -- List all available modules
  …
```

After:
```
$ panda
Usage:
  panda [--notests] [--nodeps] install [<modules> ...] -- Install the specified modules
  panda [--installed] [--verbose] list -- List all available modules
  …
```