fileio: consolidate write_string_file*()

Merge write_string_file(), write_string_file_no_create() and
write_string_file_atomic() into write_string_file() and provide a flags mask
that allows combinations of atomic writing, newline appending and automatic
file creation. Change all users accordingly.

src/rfkill/rfkill.c

@@ -127,7 +127,7 @@ int main(int argc, char *argv[]) {
                         return EXIT_SUCCESS;
                 }
 
-                r = write_string_file(saved, value);
+                r = write_string_file(saved, value, WRITE_STRING_FILE_CREATE);
                 if (r < 0) {
                         log_error_errno(r, "Failed to write %s: %m", saved);
                         return EXIT_FAILURE;