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/basic/util.c

@@ -4716,7 +4716,7 @@ int update_reboot_param_file(const char *param) {
 
         if (param) {
 
-                r = write_string_file(REBOOT_PARAM_FILE, param);
+                r = write_string_file(REBOOT_PARAM_FILE, param, WRITE_STRING_FILE_CREATE);
                 if (r < 0)
                         log_error("Failed to write reboot param to "
                                   REBOOT_PARAM_FILE": %s", strerror(-r));