MEDIUM: admin: haproxy-reload conversion to POSIX sh

MEDIUM: admin: haproxy-reload conversion to POSIX sh

The script relied on a bash-specific process substitution (< <(...)) to
feed socat's output into the read loop. This is replaced with a standard
POSIX pipe into a command group.

The response parsing is also simplified: instead of iterating over each
line with a while loop and echoing them individually, the status line is
read first, the "--" separator consumed, and the remaining output is
streamed to stderr or discarded as a whole depending on the verbosity
level.

Could be backported to 3.3 as it makes it more portable, but introduce a
slight change in the error format.