eel_v2s() now returns NULL for nil and boolean values

It used to return the constant strings "nil", "true" or "false"
for such values, which was both inconcistent (since it'll return
NULL for anything else that isn't a string or dstring) and more
troublesome than useful.
Added EEL_XSHARINGVIOLATION exception code

Replaces the not very helpful EEL_XDEVICEERROR in ShellExecute().
Restored missing memset() in st_dump()
Audiality 2 state constructor accepts nil for driver

Passing 'nil' for driver initializer (#5) is now equivalent
to leaving it out; that is, the default driver is selected.