cursor/os-compatibility: handle EINTR gracefully

If os_resize_anonymous_file() called from os_create_anonymous_file()
fails with EINTR (Interrupted system call), then the buffer allocation
fails.

To avoid that, retry posix_fallocate() on EINTR.

However, in the presence of an alarm, the interrupt may trigger
repeatedly and prevent a large posix_fallocate() to ever complete
successfully, so we need to first block SIGALRM to prevent this.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>