s390/tty3270: split up tty3270_convert_line()

s390/tty3270: split up tty3270_convert_line()

To make the code easier to read, split up tty3270_convertline()
into several subfunctions:

- tty3270_resize_line() to realloc the line if it doesn't have enough
  space left
- tty3270_required_length() to calculate how much space we need
- tty3270_add_attributes() to add the color and highlight attributes
- tty3270_add_reset_attributes() to reset the attributes at the end of
  the line

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>