Add shell_exec (and similar?) functions

[quasilyte]

They're not so useful in the server mode (arguably) but are quite handy in CLI mode.

I wouldn't forbid their use in either of the modes (server/cli), this issue is just about adding these functions to the kphp.

Research required: which functions we should add? Probably worth listing the entire shell/exec/popen related functions here in the issue for some more context.