os: Factor out CHECK_FOR_REQUIRED_ARGUMENTS
Lifted from vfb. xfree86 had almost the same thing, port it to the vfb style. Also, remove the doubled error message. I guess maybe when terminals were 9600 baud this would tell you the problem immediately instead of making you wait five seconds for the help output, so you could ^C. X11: optimizing the error path since 1987.
Signed-off-by: Adam Jackson email@example.com