kconfig: fix 'invalid option' for help option

scripts/kconfig/conf supports -? option to show the help message.
This is not wired up to Makefile, so nobody would notice this, but
it also shows 'invalid option' message.

  $ ./scripts/kconfig/conf -?
  ./scripts/kconfig/conf: invalid option -- '?'
  Usage: ./scripts/kconfig/conf [-s] [option] <kconfig-file>
  [option] is _one_ of the following:
    --listnewconfig         List new options
    --helpnewconfig         List new options and help text
    --oldaskconfig          Start a new configuration using a line-oriented program
    ...

The reason is the '?' is missing in the short option list passed to
getopt_long().

While I fixed this issue, I also changed the option '?' to 'h'.
I prefer -h (or --help, if a long option is also desired).

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>