options: use case-sensitive comparsion for options

This is better for consistency, and also allows using -V as alias
for --version.
options: add -V as alias for --version

This is a common convention.
mplayer: potentially fix main() return value

The main() function is special, and omitting the return statement would
make it always return 0. And also, mpv_main() actually never returns, it
calls exit() through exit_player() instead. But change it anyway,
because it looks misleading.
options: fix exit code when using help options

Basically a cosmetic change. Fixes github issue #88.