Expose the real mysqld exit code to process-compose, so it can report any errors. As a side-effect, removes $MYSQL_PID. Fixes #2518.

The old code used shell '&' to background mysqld, then captured its pid
with MYSQL_PID=$!, in doing so obscuring the mysqld exit code.

I can't see any sane way of both propagating the exit code *and* setting
MYSQL_PID. I also can't see any point in MYSQL_PID, given that
process-compose is a TUI and the pid is readily available in
$MYSQL_PID_FILE. So to fix the bug, I've made mysqld no longer
backgrounded (is_daemon becomes 'false').

Author: jefft

plugins/mysql/process-compose.yaml

@@ -2,8 +2,8 @@ version: "0.5"
 
 processes:
   mysql:
-    command: "mysqld --log-error=$MYSQL_HOME/mysql.log & MYSQL_PID=$! && echo 'Starting mysqld... check mysql_logs for details'"
-    is_daemon: true
+    command: "echo 'Starting mysqld... check mysql_logs for details'; mysqld --log-error=$MYSQL_HOME/mysql.log"
+    is_daemon: false
     shutdown:
       command: "mysqladmin -u root shutdown"
     availability: