Check and fix all custom BSL commands

[Christian]

Especially optional argument handling seems to be broken. numargs always gives the number of expected arguments, left out arguments have to be detected by checking

args[i].type == sl_str32 && strlen(args[i].val.value_str32) == 0

I.e. empty arguments are given as empty strings.

There might also be other errors too.

[Christian]

​Hint from Loser:

Majority of them works fine, but some are downright broken (for example "d_location") and some are malfunctioning (for example "d_powerup").