cmdline: read the "high-priority" and "clock-source" options early

The value of the command line parameter will need to be known early, when we call vlc_threads_setup().

This broke with !1274 (merged).

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v2:

• always call config_CmdLineEarlyScan() but use ifdefs for when the parameters checked should exist.

v3:

• "high-priority" is also found on OS/2
• don't do anything in config_CmdLineEarlyScan() if no parameter is going to be used