When setting the tty device to raw mode, save the VERASE character.
Later, compare incoming bytes to that.
This is somewhat complicated because "stty verase undef"
sets termios.c_cc[VERASE] = _POSIX_VDISABLE, and e.g. Linux
defines _POSIX_VDISABLE as 0 but that must not cause ELinks
to treat incoming null bytes as backspaces. Furthermore,
some systems may use different VDISABLE values for different
terminal devices, in which case _POSIX_VDISABLE is undefined
and ELinks must instead read the value from fpathconf().
49ea10a74d