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.

src/basic/chattr-util.h

@@ -34,13 +34,28 @@
          FS_NOCOW_FL        |                   \
          FS_PROJINHERIT_FL)
 
-int chattr_full(const char *path, int fd, unsigned value, unsigned mask, unsigned *ret_previous, unsigned *ret_final, bool fallback);
+typedef enum ChattrApplyFlags {
+        CHATTR_FALLBACK_BITWISE       = 1 << 0,
+        CHATTR_WARN_UNSUPPORTED_FLAGS = 1 << 1,
+} ChattrApplyFlags;
+
+int chattr_full(const char *path, int fd, unsigned value, unsigned mask, unsigned *ret_previous, unsigned *ret_final, ChattrApplyFlags flags);
+
 static inline int chattr_fd(int fd, unsigned value, unsigned mask, unsigned *previous) {
-        return chattr_full(NULL, fd, value, mask, previous, NULL, false);
+        return chattr_full(NULL, fd, value, mask, previous, NULL, 0);
 }
 static inline int chattr_path(const char *path, unsigned value, unsigned mask, unsigned *previous) {
-        return chattr_full(path, -1, value, mask, previous, NULL, false);
+        return chattr_full(path, -1, value, mask, previous, NULL, 0);
 }
 
 int read_attr_fd(int fd, unsigned *ret);
 int read_attr_path(const char *p, unsigned *ret);
+
+/* Combination of chattr flags, that should be appropriate for secrets stored on disk: Secure Remove +
+ * Exclusion from Dumping + Synchronous Writing (i.e. not caching in memory) + In-Place Updating (i.e. not
+ * spurious copies). */
+#define CHATTR_SECRET_FLAGS (FS_SECRM_FL|FS_NODUMP_FL|FS_SYNC_FL|FS_NOCOW_FL)
+
+static inline int chattr_secret(int fd, ChattrApplyFlags flags) {
+        return chattr_full(NULL, fd, CHATTR_SECRET_FLAGS, CHATTR_SECRET_FLAGS, NULL, NULL, flags|CHATTR_FALLBACK_BITWISE);
+}