Slight improvement to cursor blink timing: since the cursor doesn't

Slight improvement to cursor blink timing: since the cursor doesn't
blink when the window doesn't have focus, we don't schedule blink
timers at that point either.

Infrastructure change: term->has_focus should now not be written
directly from outside terminal.c. Instead, use the function
term_set_focus, which will sort out the blink timers as well.

[originally from svn r4911]