Re: main: fix error message for missing argument

[Paul Eggert]

Akim Demaille wrote:

> I confess I’m confused with the way program_name works: sometimes
> it’s complete, sometimes it’s the only basename (because getprogname
> and program_name are not the same).  So our error messages are
> inconsistent.

I confess I've forgotten how all that stuff works, or supposed to.

> ‘Operand’ sounds a bit weird.  I’d rather use ‘argument’.

The 'operand' terminology is taken from POSIX 
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html>. An 
operand is an argument that is neither an option nor an option-argument.