fd-util: Add fd_is_writable() to check if FD is opened for writing

fd-util: Add fd_is_writable() to check if FD is opened for writing

This checks whether a file descriptor is valid and opened in a mode that
allows writing (O_WRONLY or O_RDWR). This is useful when we want to
verify that inherited FDs can actually be used for output operations
before dup'ing them.

The helper explicitly handles O_PATH file descriptors, which cannot be
used for I/O operations and thus are never writable.