make ^A DEL work again

It was broken because we discarded all mapped sequences. Now we
let sequences with length 1 through to ProcessInput2.
extend commit #8c1b8e45, which fixed -x but forgot about -r
Amend documentation for -X option.
Fix segfault when restoring w_alt.cursor.

This fixes a crash when the window is made too small (zero height).
The crash can be reproduced by turning on altscreen and caption, and
resizing the terminal window to height = 1. After this, if the
termianl window height is increased, screen would crash when going
out of the alt screen.
Guard against NULL returns from crypt().

crypt() can return NULL on an error. Make sure these nulls are
handled properly instead of crashing. The fix is thanks to a patch
from Lukáš Nykrýn <lnykryn@redhat.com>.
avoid compiler warning about crypt
Support non-BMP utf8 characters
fix typo, duh. Thanks to Jim Mahood for reporting this.
set fontx to null instead of 0 if the encoding is switched to non-utf8

Should fix a segfault reported by consolers.
Merge remote-tracking branch 'gnu/master'