src/sysunix.c: Restart syscalls on spurious EINTR

Handle tcgetattr(), tcsetattr() and ioctl() by wrapping them in
a retry-loop and restarting them when receiving EINTR.

Should fix problem with sporadic error messages on the console
like this one:

     'Failed tcsetattr: Interrupted system call'

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>