command line: fix issues with commited code

After the patch was commited, Nadav realized that we could have an off-by-one
in the way we were handling the maximum string size. He also wanted to see the
API taking a const char instead of a char, and that is done here.

We were also leaking the string, because as far as I know, since we have an
std::vector of char *s, not std::strings, args.resize(0) won't call free.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>