Fix get_peer_tag behaviour by altering sip parsing.

The get_peer_tag parsing was broken:

Example 1:

    To: test;tag=bla

Example 2:

    To: test;tag=bla
    Other-Header: <bla>

The peer tag would not get read for example 1 because of funky parsing:

- if `>` was found, then the peer-tag would get read
- while that check was essentially created for `<test>;tag=bla`, it did not do what it was supposed to:

        ptr = strchr(to_hdr, '>');
        /* Skip past LA/RA-quoted addr-spec if any */
        if (!ptr) {
            return NULL;
        }
        ptr = strchr(to_hdr, ';'); // not using ptr, but to_hdr

Note: the other parsing functions may need to get altered too, but this is a start that fixes at least this issue. We can improve the other parsing routines later by using the new `internal_*` functions.
Note: uppercase TAG=bla is *not* recognised anymore, see the "case" comment below.
Note: display-names with `;` or `>` in it will still mess up parsing, but that's not new.
I'm not sure I understand why we'd want to seek backwards here - I'm guessing it covers the case where ptr is the newline after a header, but it's not clearly documented what the behaviour of this method is in that case.