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/path-util.h

@@ -131,7 +131,7 @@ int parse_path_argument_and_warn(const char *path, bool suppress_root, char **ar
 char* dirname_malloc(const char *path);
 
 bool filename_is_valid(const char *p) _pure_;
-bool path_is_safe(const char *p) _pure_;
+bool path_is_normalized(const char *p) _pure_;
 
 char *file_in_same_dir(const char *path, const char *filename);