Checks the specified file to see if it is executable
or if by appending one of the
+executableExtensions
it can be made executable. The return value is the
actual executable path or nil if the
file cannot be executed.
Returns the launch path for a tool given the
name of a tool. Locates the tool by
looking in the standard directories and, if not
found there, looking in the PATH set in the
environment. On ms-windows, this also
tries appending common executable path extensions to
the tool name in order to find it.
Returns the path found, or nil if the
tool could not be located.