ftp: don't use restartable signals

Refactor to not rely upon restartable signals (SA_RESTART),
possibly fixing intermittent failures with -q QUITTIME.

ftp transfers: handle EINTR/EAGAIN in copy_bytes(),
instead of relying upon restartable signals.

http/https transfers: Explicitly print an error similar to
progressmeter() when timing-out for -Q QUITTIME in fetch_wait(),
and set errno to ETIMEDOUT so that the warn() in fetch_url()
prints a more accurate error message.

PR/55857

(lukem)