Revisit fstrm_capture --max-conns concurrent connections

[reedjc]

Document what numbers are allowed for --maxconns for fstrm_capture.

Check for valid max-conns value. (Because libmy/argv.c using atoi(3) has many caveats and doesn't do checking).

It defaults to -1 and each cb_accept_conn will decrement (like == -2) and cb_close_conn will increase (like == 0). I am unclear if this behavior is correct. And I didn't test it. cmikk asked me to open this ticket to revisit its behavior, such as if the 0 is used.