terminal-util: when resetting terminals, don't wait for carrier

terminal-util: when resetting terminals, don't wait for carrier

In case of non-CLOCAL lines (i.e. those with carrier detect configured)
we shouldnt wait for a carrier if all we try to do is reset the TTY.
Hence, whenever we open such a TTY pass O_NONBLOCK.

Note that we continue to open ttys we intend to write to without
O_ONBLOCK, we only add it in cases we invoke ioctl()s or other terminal
operations without reading or writing to the device.

Fixes #835.