kconfig: use getopt() in conf.c for handling command line arguments

Switch from doing our own parsing of command line arguments to
using getopt(3) to do it.  Aside from simplifying things, this allows us to
specify multiple arguments; the old code could only accept two arguments
(input_mode and kconfig name).

Note some subtle changes:
 - The argument '-?' is no longer supported.
 - '-h' is not treated as an error, so output goes to stdout, and we
   exit with '0'.
 - There is no compatibility checking amongst arguments; the last option
   will simply override earlier options.  For example, 'conf -n -y foo'
   is perfectly valid now (input_mode will be set_yes).  Previously, that
   would have been an error ("can't find file -y").

Signed-off-by: Andres Salomon <dilinger@debian.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Roman Zippel <zippel@linux-m68k.org>