[MacPorts] #67955: startupitem: enhancements: logfile should also log to stderr; add user/group capabilities

[MacPorts]

#67955: startupitem: enhancements: logfile should also log to stderr; add
user/group capabilities
-------------------------+--------------------
 Reporter:  mascguy      |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:  2.8.1
 Keywords:  startupitem  |       Port:
-------------------------+--------------------
 A few suggested enhancements, which would further improve `startupitem`
 functionality:

 * `startupitem.logfile`: This option only enables stdout logging, via
 `StandardOutPath`. It should also log stderr, via `StandardErrorPath`.
 * Presently, there is no built-in capability to support a custom user
 and/or group, requiring each port to implement this via various means.
 (Generally either via `su`, or a custom plist file.) But ideally, a port
 should be able to enable such functionality via optional properties. (Say,
 `startupitem.run_user` and `startupitem.run_group`, or something similar.)

 Some ideas/assumptions about the latter:
 * Specifying a custom user and/or group, would not be supported with a
 custom plist file.
 * Depending on macOS version support, this could potentially be
 accomplished at the `launchd` level, via properties `GroupName` and
 `UserName`. Alternatively, it could be done by wrapping commands via `su`.
 (Or whichever method is most appropriate, to support the full swath of
 macOS releases.)

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Ticket URL: <https://trac.macports.org/ticket/67955>
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