Fix issue with jsexec being used for CGI scripts

If jsexec is used for a CGI script and the SBBSCTRL env var was not set, a warning would be printed (to stderr), even if the -c option was used to specify the path to the ctrl dir.

get_ctrl_dir() now accepts a boolean argument to specify whether to print a warning when the default value ("/sbbs/ctrl") is used. Pass FALSE for jsexec, TRUE for other callers (currently).