logging: avoid changing errno.
logging: avoid changing errno.

So user can safely log syscall results without worrying about errno changing.

markokr/libusual,ifduyue/libusual,ifduyue/libusual,markokr/libusual,markokr/libusual,rabbitt/libusual,rabbitt/libusual,markokr/libusual,rabbitt/libusual,mgjv/libusual,rabbitt/libusual,mgjv/libusual,mgjv/libusual,ifduyue/libusual,ifduyue/libusual,mgjv/libusual