Function command_on_path 

pub fn command_on_path(cmd: &str) -> bool

What: Determine whether a command is available on the Unix PATH.

Inputs:

• cmd: Program basename or path containing a path separator.

Output:

• true when an executable file is found with the executable bit set.

Details:

• Honours Unix permission bits so a non-executable file on PATH is not treated as a tool.
• Ported from Pacsea’s install/utils.rs.