string-arguments of functions in 'std.file'

[CLXX]

dmd2.025, Windows

Some standard functions receive 'string' arguments.
This is not a bug, but not good specs.

void[] read( in string name );
void[] write( in string name, const void[] buffer );
...

They should be

void[] read( in char[] name );
void[] write( in char[] name, const void[] buffer );
...

Former cannot receive char[]-arguments, and latter can.
This change has backward compatibility.