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).