fileio: port write_string_file() to LabelOps, and thus add WRITE_STRING_FILE_LABEL flag

Given that we have the LabelOps abstraction these days, we can teach
write_string_file() to use it, which means we can get rid of
fileio-label.[ch] as a separate concept.

(The only reason that fileio-label.[ch] exists independently of
fileio.[ch] was that the former linekd to libselinux potentially, and
thus had to be in src/shared/ while the other always was in src/basic/.
But the LabelOps vtable provides us with a nice work-around)

src/shared/dropin.c

@@ -12,7 +12,7 @@
 #include "dropin.h"
 #include "escape.h"
 #include "fd-util.h"
-#include "fileio-label.h"
+#include "fileio.h"
 #include "hashmap.h"
 #include "log.h"
 #include "macro.h"
@@ -87,7 +87,7 @@ int write_drop_in(
         if (r < 0)
                 return r;
 
-        return write_string_file_at_label(AT_FDCWD, p, data, WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_ATOMIC|WRITE_STRING_FILE_MKDIR_0755);
+        return write_string_file(p, data, WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_ATOMIC|WRITE_STRING_FILE_MKDIR_0755|WRITE_STRING_FILE_LABEL);
 }
 
 int write_drop_in_format(

src/shared/fileio-label.c

@@ -6,20 +6,6 @@
 #include "fileio.h"
 #include "selinux-util.h"
 
-int write_string_file_full_label(int atfd, const char *fn, const char *line, WriteStringFileFlags flags, struct timespec *ts) {
-        int r;
-
-        r = mac_selinux_create_file_prepare_at(atfd, fn, S_IFREG);
-        if (r < 0)
-                return r;
-
-        r = write_string_file_full(atfd, fn, line, flags, ts);
-
-        mac_selinux_create_file_clear();
-
-        return r;
-}
-
 int create_shutdown_run_nologin_or_warn(void) {
         int r;
 
@@ -33,9 +19,10 @@ int create_shutdown_run_nologin_or_warn(void) {
          * 13 years later we stopped managing /etc/nologin, leaving it for the administrator to manage.
          */
 
-        r = write_string_file_atomic_label("/run/nologin",
-                                           "System is going down. Unprivileged users are not permitted to log in anymore. "
-                                           "For technical details, see pam_nologin(8).");
+        r = write_string_file("/run/nologin",
+                              "System is going down. Unprivileged users are not permitted to log in anymore. "
+                              "For technical details, see pam_nologin(8).",
+                              WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_ATOMIC|WRITE_STRING_FILE_LABEL);
         if (r < 0)
                 return log_error_errno(r, "Failed to create /run/nologin: %m");
 

src/shared/fileio-label.h

@@ -9,12 +9,4 @@
 
 #include "fileio.h"
 
-int write_string_file_full_label(int atfd, const char *fn, const char *line, WriteStringFileFlags flags, struct timespec *ts);
-static inline int write_string_file_at_label(int atfd, const char *fn, const char *line, WriteStringFileFlags flags) {
-        return write_string_file_full_label(atfd, fn, line, flags, /* ts= */ NULL);
-}
-static inline int write_string_file_atomic_label(const char *fn, const char *line) {
-        return write_string_file_at_label(AT_FDCWD, fn, line, WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_ATOMIC);
-}
-
 int create_shutdown_run_nologin_or_warn(void);