main: rework CLI frontend

 - Change command line flags to be align better with those of other
   interpreters and with the gcc compiler, e.g. `-D` and `-U` to
   define and undefine globals, `-e` to execute script expression etc.

 - Pass only excess CLI arguments as `ARGV` to scripts, e.g.
   `ucode -e 'print("Hello world")' -- -x -y` would pass only
   `[ "-x", "-y" ]` as ARGV contents

 - Default to raw mode and introduce flag to enable template mode

Signed-off-by: Jo-Philipp Wich <jo@mein.io>