Use upsdebug_with_errno() instead of a few cases calling upsdebugx(..., errno, strerror()) explicitly

[jimklimov]

We have a nice helper method and a handful of places that can be made more simple with this change.

Single-line hits are easy to find with git grep (there may be more with multi-line code involved):

:;  git grep -E upsdebugx'.*(strerror|errno)' | grep -v _strerror
clients/upssched.c:                     upsdebugx(6, "read() from fd %d returned %" PRIiSIZE " (bytes): '%c'; errno=%d: %s",
clients/upssched.c:                                     upsdebugx(4, "read() from fd %d returned 0; errno=%d: %s",
drivers/blazer_ser.c:           upsdebugx(3, "send: %s", ret ? strerror(errno) : "timeout");
drivers/blazer_ser.c:           upsdebugx(3, "read: %s", ret ? strerror(errno) : "timeout");
drivers/main.c:                         upsdebugx(1, "Socket dialog with the other driver instance: %s", strerror(errno));
drivers/main.c:                 upsdebugx_with_errno(1, "Socket dialog with the other driver instance");
drivers/riello_usb.c:                   upsdebugx(3, "USB: Send_USB_Packet: send_usb_packet, err = %d %s ", err, strerror(errno));
drivers/riello_usb.c:                   upsdebugx(3, "USB: Send_USB_Packet: send_usb_packet, err = %d %s ", err, strerror(errno));
drivers/riello_usb.c:           upsdebugx(3, "USB: Get_USB_Packet: send_usb_packet, err = %d %s ", err, strerror(errno));

(Note we have quite a few helpers like nut_usb_strerror, shut_strerror, modbus_strerror... which do not easily fall into this cleanup's targets, but might benefit from similar helpers).