Restore -detached argument detection in rabbitmq-server

Commit 68c3055 removed the code that
detected the `-detached` command-line argument and set the `detached`
variable. However, the conditional check for `$detached` remained, along
with a comment referencing "the code above" that no longer exists. This
left orphaned code that could never execute its intended branch.

This change restores the argument detection loop that sets `detached` to
`"true"` when `-detached` is found in the command-line arguments. The
loop uses POSIX-compliant syntax to iterate through all positional
parameters and check for the `-detached` flag.

Additionally, this change replaces the deprecated `-o` operator with
`||` in the conditional test, following POSIX best practices for boolean
OR operations in shell scripts.

(cherry picked from commit a48ea81)
(cherry picked from commit 6218d57)

Author: lukebakken

deps/rabbit/scripts/rabbitmq-server

@@ -16,6 +16,12 @@ SCRIPTS_DIR=$(dirname "$0")
 
 [ "$NOTIFY_SOCKET" ] && RUNNING_UNDER_SYSTEMD=true
 
+for opt in "$@"; do
+    if [ "$opt" = "-detached" ]; then
+        detached="true"
+    fi
+done
+
 RABBITMQ_DEFAULT_ALLOC_ARGS="+MBas ageffcbf +MHas ageffcbf +MBlmbcs 512 +MHlmbcs 512 +MMmcs 30"
 
 check_start_params() {
@@ -94,13 +100,12 @@ stop_rabbitmq_server() {
     fi
 }
 
-if [ "$RABBITMQ_ALLOW_INPUT" -o "$RUNNING_UNDER_SYSTEMD" -o "$detached" ]; then
-    # Run erlang VM directly, completely replacing current shell
-    # process - so the pid file written in the code above will be
-    # valid (unless detached, which is also handled in the code
-    # above).
+# Note: the docker-library/rabbitmq#778 change exports "RUNNING_UNDER_SYSTEMD=true" to start
+# the erl process using exec via the start_rabbitmq_server.
+if [ "$RABBITMQ_ALLOW_INPUT" ] || [ "$RUNNING_UNDER_SYSTEMD" ] || [ "$detached" ]; then
+    # Run erlang VM directly, completely replacing current shell process
     #
-    # And also this is the correct mode to run the broker under
+    # This is also the correct mode to run the broker under
     # systemd - there is no need in a proxy process that converts
     # signals to graceful shutdown command, the unit file should already
     # contain instructions for graceful shutdown. Also by removing