fs-util: rename path_is_safe() → path_is_normalized()

Already, path_is_safe() refused paths container the "." dir. Doing that
isn't strictly necessary to be "safe" by most definitions of the word.
But it is necessary in order to consider a path "normalized". Hence,
"path_is_safe()" is slightly misleading a name, but
"path_is_normalize()" is more descriptive, hence let's rename things
accordingly.

No functional changes.

src/basic/user-util.c

@@ -605,7 +605,7 @@ bool valid_home(const char *p) {
         if (!path_is_absolute(p))
                 return false;
 
-        if (!path_is_safe(p))
+        if (!path_is_normalized(p))
                 return false;
 
         /* Colons are used as field separators, and hence not OK */