[syslinux] correct "--help' options

[Ady Ady]

Hello hpa,
 
When running each ldlinux.sys installer:
extlinux
syslinux
syslinux[64].exe
syslinux.com
 
 with the "--help" argument, the results are different, depending on
 the specific installer.
 
Under the Linux-based installer(s):
  --once=...       Execute a command once upon boot
 
while for the others:
  --once=...   -o  Execute a command once upon boot
 
Note that the "-o" is missing under Linux.
 
I haven't tested whether "-o" is actually working or not in any of the
 installers. It would be convenient to test and unify both:
_ the actual behavior of  "<installer> -o", and;
_ the "--help" screen.
 
A similar example is
  --directory  -d  Directory for installation target
 
which appears after "syslinux --help" but not after "extlinux --help".
Does extlinux support "--directory"?
 
In addition to those requests, I'd like to know if there is any specific
 "logic" for the order of appearance of the options when running
 "<installer> --help".
 
TIA,
Ady.