systemd-nspawn: make SettingsMask 64 bit wide

systemd-nspawn: make SettingsMask 64 bit wide

The use of UINT64_C() in the SettingsMask enum definition is misleading:
it does not mean that individual fields have this width. E.g., with
enum {
   FOO = UINT64_C(1)
}
sizeof(FOO) gives 4. It only means that the shift is done properly. So
1 << 35 is undefined, but UINT64_C(1) << 35 is the expected 64 bit
constant. Thus, the use UINT64_C() is useful, because we know that the shifts
are done properly, no matter what the value of _RLIMIT_MAX is, but when those
fields are used in expressions, we don't know what size they will be
(probably 4). Let's add a define which "hides" the enum definition behind a
define which gives the same value but is actually 64 bit. I think this is a
nicer solution than requiring all users to cast SETTING_RLIMIT_FIRST before
use.

Fixes #9035.