iovec-util: rework IOVEC_MAKE_STRING() to work with compound initialized input

iovec-util: rework IOVEC_MAKE_STRING() to work with compound initialized input

This avoids the ({}) that IOVEC_MAKE_STRING() so far used and might
cause a memory corruption if the parameter passed in is itself allocated
via a compount initialized array or so.

Also, this makes sure both IOVEC_MAKE_STRING() and IOVEC_MAKE() accept
'const' parameters without this causing a compiler warning.