Expose the real mysqld exit code to process-compose, so it can report any errors. Fixes #2518 (#2519)

## Summary

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 (hence `is_daemon` becomes 'false').

## How was it tested?

Testing is as in the description of #2518: set up mysqld to fail (e.g.
claim port 3306) and `devbox services up`.

The mysqld process *ought* to go into a failure loop, toggling between
status 'Restarting' and 'Running' and setting exit code 1.

(It's not related to this patch, but I suggest also limiting these
infinite restarts by setting `max_restarts`, e.g.:
```
    availability:
      restart: "always"
      max_restarts: 5
```
)

---------

Co-authored-by: John Lago <[email protected]>

Author: jefft

plugins/mariadb/process-compose.yaml

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

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: