tc_unix.go: Make cfmakeraw compatible to Linux

[ktock]

According to termios(3), cfmakeraw should perform or on c_cflag with CS8.
But the current our cfmakeraw implementation doesn't do that but clears that bit.
This commit fixes cfmakeraw to make it behave same as Linux's one.

https://man7.org/linux/man-pages/man3/termios.3.html

           termios_p->c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP
                           | INLCR | IGNCR | ICRNL | IXON);
           termios_p->c_oflag &= ~OPOST;
           termios_p->c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
           termios_p->c_cflag &= ~(CSIZE | PARENB);
           termios_p->c_cflag |= CS8;

[andrewsun2898]

Quick ping on this. We're running into some issues with switching into raw mode that should be fixed with this pr. Can we get this submitted?