s390/con3270: Use strscpy() instead of strcpy()

Use strscpy() instead of strcpy() so that bounds checking is performed
on the destination buffer. This requires to keep track of the size of
the dynamically allocated prompt memory area, which is done with a new
prompt_sz within struct tty3270.

Reviewed-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>