while here mention the codeberg mirror on the web version too

these are unused after the fs/ui merge.

while here also do more validation in IMSG_GET_RAW and add a missing NULL check in the return type of req_by_id in the IMSG_CERT_STATUS.

just declare the same function in the two branches of #if HAVE_ASR_RUN to avoid further #ifs

rework mark_nonblock so it sets the cloexec flag too and use it in control.c to avoid these flags. (which are expected to become available on a future revision of POSIX and are already widely available.) It's not an issue for telescope to do the socket/accept + fcntl dance because there aren't threads that can fork(2) (there are no threads at all!) reported by @sikmir at github https://github.com/omar-polo/telescope/commit/59ef79dd19611c7846b00427e6f2267c748ae290#r74498414