linux-user: Fix broken "-version" option

Fix the "-version" option, which was accidentally broken in commit
fc9c541:
 * exit after printing version information rather than proceeding
   blithely onward (and likely printing the full usage message)
 * correct the cut-n-paste error in the usage message for it
 * don't insist on the presence of a following argument for
   options which don't take an argument (this was preventing
   'qemu-arm -version' from working)
 * remove a spurious argc check from the beginning of main() which
   meant 'QEMU_VERSION=1 qemu-arm' didn't work.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andrzej Zaborowski <andrew.zaborowski@intel.com>
