URL: fix ASCII dependency in strcpy_url and strlen_url

Commit 3c630f9b partially reverted the
changes from commit dd7521bc because of
the problem that strcpy_url() was modified unilaterally without also
modifying strlen_url(). As a consequence strcpy_url() was again
depending on ASCII encoding.

This change fixes strlen_url() and strcpy_url() in parallel to use a
common host-encoding independent criterion for deciding whether an URL
character must be %-escaped.

Closes #2535