Error condition hierarchy and accessors need fixes

[Stelian Ionescu]

iolib:socket-error and iolib.multiplex:poll-error both contain FDs, but the their accessors are different: isys:handle-of and iolib.multiplex:poll-error-fd. Perhaps poll-error should inherit from syscall-error ?
Furthermore, streams use fd-of and there's no reason to have 3 different accessors for this