Issue #8597 shows Valgrind result indicating a memory leak in
InteractiveConsole. This is fixed in #8619 by ensuring that free() is
called appropriately.
This commit takes that a step further by removing all manual memory
management in favour of using vector<string>.
- argc is now gone in favour of argv.empty()/argv.size()
- argv_t is a vector of strings of utf8's
- console_command_func's signature is changed accordingly