tree-wide: use UTMPX_FILE rather than _PATH_UTMPX

Apparently _PATH_UTMPX is a glibc'ism. UTMPX_FILE is the same thing and
what everyone else uses. Since they are otherwise equivalent, let's just
switch.

src/shared/utmp-wtmp.c

@@ -42,7 +42,7 @@ int utmp_get_runlevel(int *runlevel, int *previous) {
                 return 0;
         }
 
-        if (utmpxname(_PATH_UTMPX) < 0)
+        if (utmpxname(UTMPX_FILE) < 0)
                 return -errno;
 
         utmpx = utxent_start();
@@ -91,7 +91,7 @@ static int write_entry_utmp(const struct utmpx *store) {
          * each entry type resp. user; i.e. basically a key/value
          * table. */
 
-        if (utmpxname(_PATH_UTMPX) < 0)
+        if (utmpxname(UTMPX_FILE) < 0)
                 return -errno;
 
         utmpx = utxent_start();

src/shared/wall.c

@@ -51,7 +51,7 @@ static int wall_utmp(
 
         /* libc's setutxent() unfortunately doesn't inform us about success, i.e. whether /var/run/utmp
          * exists. Hence we have to check manually first. */
-        if (access(_PATH_UTMPX, F_OK) < 0) {
+        if (access(UTMPX_FILE, F_OK) < 0) {
                 if (errno == ENOENT)
                         return -ENOPROTOOPT;