libcdecl: Avoid vsnprintf for error reporting.

libcdecl: Avoid vsnprintf for error reporting.

Using vsnprintf is overkill here.  We only need to handle a single %s
conversion, which can be done by direct call to snprintf.

As this is the only caller of vsnprintf, dropping it means we can drop
the vsnprintf gnulib module.  Also, at least with gcc, using va_start
and such is fairly expensive.  The direct-snprintf version is quite
a bit more compact.