perf ui: Improve handling sigwinch a bit

No need to unblock it at each ui__getch() and also allow other users to
check if a resize is needed, or force an refresh of terminal dimensions.

The 'force' one shouldn't be needed, but its in a slow path, so leave it
like that for now, I'll revisit this another day.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-aujchu6yx3bfy64non1rky0w@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>