basic/terminal-util: operate on one fd in get_default_background_color()

basic/terminal-util: operate on one fd in get_default_background_color()

This moves the open call earlier, so that we do any state-changing operations
if we actually managed to open the nonblocking fd. The code is easier to follow
this way and might be more robust.

Suprisingly, this fixes https://github.com/systemd/systemd/issues/39055: it
seems that run0 chowns /dev/stdin (in my case /dev/pts/0) to root:root, and the
second run0 can read and write stdin/stdout throught the already-open fds,
but fd_reopen fails.

Fixes https://github.com/systemd/systemd/issues/39055.