core: block to get good random bytes for "/var/lib/NetworkManager/secret_key"

_host_id_read() is the only place where we really care to have good
random numbers, because that is the secret key that we persist to disk.

Previously, we tried only nm_random_get_bytes_full(), which is a best
effort to get strong random numbers. If it fails to generate those,
it would simply remember the generated key in memory and proceed, but not
persist it to disk.

nm_random_get_bytes_full() does not block waiting for good numbers.

Change that. Now, first call nm_random_get_crypto_bytes(), which would
block and try hard to get good random numbers. Only if that fails,
fallback to nm_random_get_bytes_full() as before. The difference is of
course only in early boot, when we might not yet have entropy. In that
case, I think it's better for NetworkManager to block.
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