treewide: sanitize loop_write

loop_write() didn't follow the usual systemd rules and returned status
partially in errno and required extensive checks from callers. Some of
the callers dealt with this properly, but many did not, treating
partial writes as successful. Simplify things by conforming to usual rules.
ima-setup: simplify
systemctl: fix invalid free when enabling sysv services fails

The error was introduced in v215-343-g60731f32f1 'systemctl: do not
bother to mutate state on error', by causing strv_free to attempt to
free a static string. Simplify the whole thing by always keeping the
array in valid state.