manpages: portfile: document keywords startupitem.user/startupitem.group

[mascguy]

Add info for keywords startupitem.user and startupitem.group, which are presently undocumented:

     startupitem.user
         Run the daemon via the specified user.
         Type: optional
         Default: none
         Values: username
         Example:
               startupitem.user my_daemon_username

     startupitem.group
         Run the daemon via the specified group.
         Type: optional
         Default: none
         Values: groupname
         Example:
               startupitem.group my_daemon_groupname

If anyone would like to add/change the description, please speak up ASAP. Otherwise let's merge this, and at least ensure they're included in our documentation. (We can always refine this later, if folks want to add more.)

Corresponding PR, for the MacPorts Guide:

PR 59 - guide: startupitem: document keywords startupitem.user/startupitem.group

[mascguy]

Related to PR: PR 202 - portstartupitem: add startupitem options so that ports can specify the user and group to run a launchd service as