chattr-util: generalize chattr manipulation for files with secrets from journalctl

chattr-util: generalize chattr manipulation for files with secrets from journalctl

This moves the code for setting chattr file attributes appropriate for
"secrets" files from journalctl into generic chattr-util.c code so that
we can use it elsewhere.

Also, let's reuse the "bitwise" logic already implemented in the chattr
code, instead of doing it again.