tree-wide: add path_simplify_alloc() and use it

path_simplify_full()/path_simplify() are changed to allow a NULL path, for
which a NULL is returned. Generally, callers have already asserted before that
the argument is nonnull. This way path_simplify_full()/path_simplify() and
path_simplify_alloc() behave consistently.

In sd-device.c, logging in device_set_syspath() is intentionally dropped: other
branches don't log.

In mount-tool.c, logging in parse_argv() is changed to log the user-specified
value, not the simplified string. In an error message, we should show the
actual argument we got, not some transformed version.

src/basic/user-util.c

@@ -589,7 +589,6 @@ int getgroups_alloc(gid_t** gids) {
 int get_home_dir(char **ret) {
         struct passwd *p;
         const char *e;
-        char *h;
         uid_t u;
 
         assert(ret);
@@ -622,18 +621,12 @@ int get_home_dir(char **ret) {
                 return -EINVAL;
 
  found:
-        h = strdup(e);
-        if (!h)
-                return -ENOMEM;
-
-        *ret = path_simplify(h);
-        return 0;
+        return path_simplify_alloc(e, ret);
 }
 
 int get_shell(char **ret) {
         struct passwd *p;
         const char *e;
-        char *s;
         uid_t u;
 
         assert(ret);
@@ -665,12 +658,7 @@ int get_shell(char **ret) {
                 return -EINVAL;
 
  found:
-        s = strdup(e);
-        if (!s)
-                return -ENOMEM;
-
-        *ret = path_simplify(s);
-        return 0;
+        return path_simplify_alloc(e, ret);
 }
 
 int reset_uid_gid(void) {