syscall.c: add support for printing return value and error code

syscall.c: add support for printing return value and error code

Some weird syscalls (like modify_ldt) return value that doesn't indicate
an error (less than -4096), but it is, in fact, erroneous (because they
decide to clip the return value to 32-bit, for example).

Add a flag to print decoded error code along with syscall return value.

* defs.h (RVAL_PRINT_ERR_VAL): New rval flag.
* syscall.c (syscall_exiting_trace): Handle it.