This commit makes the shell:
- highlight executables in the current directory as invalid, unless an
explicit `./' is given (so, `./foo` isn't red, but `foo` is)
- not suggest executables in the current directory unless explicitly
requested (by prepending `./`)
- not attempt to run an executable in the current directory that has
been invoked as a program name and failed execvp().
Note that `./foo` is still executed because it's not invoked as
a name, but rather as a path.
Fixes the other half of #6774.
b7fb12338c