[PR fortran/35832] Give better error messages for bad IO-tags

[Tobias Schlüter]

Hi,

this straightforward patch gives better error messages for malformed I/O 
statements, by parsing them in smaller parts.  With this patch, 
something like
    ACCESS=123
in an I/O statement is not rejected in its entirety as a syntax error, 
but instead an error like
    Invalid value for ACCESS specification
is printed.

Built and tested on i386-darwin.  Ok?

Cheers,
- Tobi