Feature request: add method to unix::process::CommandExt to set argv[0]

[jsgf]

In Unix, the first process argument is the executable name by convention. However, there are various occasions where you might want to set argv[0] to something other than the full executable path:

• to set a normalized program name
• to put multiple functions into one executable, selected by argv[0]
• to put commentary on the execution context for a process
• etc

I suggest adding a set_argv_0<S: AsRef<OsStr>(&mut self, arg: S) -> process::Command to std::os::unix::process::CommandExt to allow argv[0] to be overridden.

The default behaviour would remain unchanged.

It would require a little more implementation change - currently it always executes self.get_argv()[0], but this should be changed to self.program.as_ptr().

[jsgf]

cc @bolinfest

[bstrie]

This can be closed in light of #66512 , correct?

[jsgf]

yup