Ignore EPERM when modifying kern.geom.debugflags

Ignore EPERM when modifying kern.geom.debugflags

Many tests fail when run as a non-root user on FreeBSD.  The failures
all amount to an inability to open files using grub_util_fd_open,
because we cannot set the kern.geom.debugflags sysctl.  This sysctl is
indeed important to allow us to do such things as installing GRUB to the
MBR, but if we need to do that and can't then we will get an error
later.  Enforcing it here is unnecessary and prevents otherwise
perfectly reasonable operations.