Introduce, and start using, tvb_ip_to_str() and tvb_ip6_to_str(). These

Introduce, and start using, tvb_ip_to_str() and tvb_ip6_to_str().  These
do the same as the non-tvb equivalents but take a TVB and an offset instead
of a pointer to an array of bytes.

Their purpose is to prevent (many) dissectors from doing:
ip_to_str(tvb_get_ptr(...)).

(About the names and the location: I like the names as they are but the names
imply that they should live in tvbuff.c.  That would make some sense but
I didn't want to pull to_str.h into tvbuff.c...)

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@35519 f5534014-38df-0310-8fa8-9805f1628bb7