Allow to use glslmin with the system() function

It is currently not possible to use glslmin with the system() function
(or anything else which does not have a tty) without using a pipe
(`cat file | glslmin`), since it will never map the arguments to a
stream if the input is not a tty.

This commit fixes this issue by always mapping the arguments to
streams, and fallback on stdin if there is no argument (the previous
behavior was to display the help message).