creds-util: fix "weak" vs. "secure" display for tmpfs/noswap backed credentials

creds-util: fix "weak" vs. "secure" display for tmpfs/noswap backed credentials

When we display passed credentials we show a brief safety level based on
how the credential is pass in: if it's backed by swappable memory we
give it a "weak" level. This check was so far done by checking if the
file is backed by ramfs. However, since
1155f44f48f8fd59c863d71b3938e34a0b2fec2a we actually prefer tmpfs with
the new "noswap" option for this.

Hence, fix this, and explicitly look for "noswap" among the mount
options in case we detect tmpfs.