rename clean to clean-current, and clean-all back to clean

This way, the clean target continues to do what it always did: delete
the whole build directory.  You can use clean-current to just delete
the currently-configured build directory.
fix SIGSEGV and off-by-one error in logDebug

We must use separate va_start/va_end pairs for each call to vsnprintf
on Linux and possibly other platforms in order to avoid a crash.
Also, we need to give it room to null terminate the string at the
right point.