startupitem: enhancements: logfile should also log to stderr; add user/group capabilities

[mascguy (Christopher Nielsen)]

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.)