Make fopen_temporary and fopen_temporary_label unlocked

This is partially a refactoring, but also makes many more places use
unlocked operations implicitly, i.e. all users of fopen_temporary().
AFAICT, the uses are always for short-lived files which are not shared
externally, and are just used within the same context. Locking is not
necessary.

src/resolve/resolved-link.c

@@ -2,7 +2,6 @@
 
 #include <net/if.h>
 #include <linux/if.h>
-#include <stdio_ext.h>
 #include <unistd.h>
 
 #include "sd-network.h"
@@ -1178,7 +1177,6 @@ int link_save_user(Link *l) {
         if (r < 0)
                 goto fail;
 
-        (void) __fsetlocking(f, FSETLOCKING_BYCALLER);
         (void) fchmod(fileno(f), 0644);
 
         fputs("# This is private data. Do not parse.\n", f);

src/resolve/resolved-resolv-conf.c

@@ -1,7 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
 #include <resolv.h>
-#include <stdio_ext.h>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
@@ -359,14 +358,12 @@ int manager_write_resolv_conf(Manager *m) {
         if (r < 0)
                 return log_warning_errno(r, "Failed to open private resolv.conf file for writing: %m");
 
-        (void) __fsetlocking(f_uplink, FSETLOCKING_BYCALLER);
         (void) fchmod(fileno(f_uplink), 0644);
 
         r = fopen_temporary_label(PRIVATE_STUB_RESOLV_CONF, PRIVATE_STUB_RESOLV_CONF, &f_stub, &temp_path_stub);
         if (r < 0)
                 return log_warning_errno(r, "Failed to open private stub-resolv.conf file for writing: %m");
 
-        (void) __fsetlocking(f_stub, FSETLOCKING_BYCALLER);
         (void) fchmod(fileno(f_stub), 0644);
 
         r = write_uplink_resolv_conf_contents(f_uplink, dns, domains);