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).