Add screen locking calls to sc cn grab and ungrab. The locking functions
just use the same mutex locking as sc cn putc so they have the same
defects.

The locking calls to acquire the lock are actually in sc cn open and close.
Ungrab has to unlock, although this opens a race window.

Change the direct mutex lock calls in sc cn putc to the new locking
functions via the open and close functions. Putc also has to unlock, but
doesn't keep the screen open like grab. Screen open and close reduce to
locking, except screen open for grab also attempts to switch the screen.

Keyboard locking is more difficult and still null, even when keyboard
input calls screen functions, except some of the functions have locks
too deep to work right.

This organization gives a single place to fix some of the locking.