Add/use a new get_ctrl_dir() function to query the SBBSCTRL env var and if
not set (or blank), use the default ("/sbbs/ctrl") and print a warning to stderr. This resolves a FAQ for sysops that struggle with setting the SBBSCTRL env var, but do have sbbs install in /sbbs or a symlink from /sbbs to where sbbs is actually installed. Note: this function is different than the one previously defined in getctrl.c but never used. If/when we ever really need to support PREFIX, then perhaps we'll use that one (which copies a string to a buffer).