xorg-server does not follow the convention of using 2 dashes.

Note that --help does produce the help output. However, it
produces this additional output after it:

Error: Aborting session with 'Unrecognized option: --help'.
Session: Aborting session at 'Sun Jun 28 01:35:35 2015'.
Session: Session aborted at 'Sun Jun 28 01:35:35 2015'.

It also causes a return code of 1 rather than 0.

Therefore, we should instruct users to call -help instead.

v2: backport to nx-libs 3.5.0.x (Mihai Moldovan)

Affects:
  - 0009_nxagent_add-man-page.full.patch
  - 0009_nxproxy_add-man-page.full+lite.patch
  - 0209_x2goagent_add-man-page.full.patch