In fragment identifiers, use ~%d for ordinal suffixes,

and reserve the character '~' for that purpose.

Bug found by validator.w3.org in openssl(1), which contains both a
tag "tls1_2" and a second instance of a tag "tls1", which also resulted
in "tls1_2", causing a clash.  Now, the second instance of "tls1" is
rendered as "tls1~2" instead, employing the newly reserved '~'.