spacecheck: verify filename lengths and characters

Maximum filename length 64, of which 48 maximum for the filename part.
Allowed characters: `A-Za-z0-9/._-`.

Also:
- rename a file to pass the check.
- init max constants outside the loop.
- minor fix to an error message.
  Follow-up to 62d77b12fc #21087

Closes #21095

docs/Makefile.am

@@ -68,7 +68,7 @@ INTERNALDOCS =                                   \
   internals/SCORECARD.md                         \
   internals/SPLAY.md                             \
   internals/STRPARSE.md                          \
-  internals/THRDPOOL+QUEUE.md                    \
+  internals/THRDPOOL-AND-QUEUE.md                \
   internals/TIME-KEEPING.md                      \
   internals/TLS-SESSIONS.md                      \
   internals/UINT_SETS.md                         \