tty: add the option to have a tty reject a new ldisc

In the Linux kernel, the following vulnerability has been resolved:

tty: add the option to have a tty reject a new ldisc

... and use it to limit the virtual terminals to just N_TTY. They are

kind of special, and in particular, the "con_write()" routine violates

the "writes cannot sleep" rule that some ldiscs rely on.

This avoids the

BUG: sleeping function called from invalid context at kernel/printk/printk.c:2659

when N_GSM has been attached to a virtual console, and gsmld_write()

calls con_write() while holding a spinlock, and con_write() then tries

to get the console lock.