struct/dstr.[ch3] (dstr_putc): Accept an `int' argument.

This is a rather annoying problem.  It's useful to store binary data in
`dstr' objects, and, indeed, characters encoded (as <stdio.h> does) as
`unsigned char' values in an `int' object; but the `char' type of the
argument requires an implicit conversion, which may cause undefined
behaviour, and raises warnings if called with an out-of-range constant
value.