Moved various to_str files from packet.{c,h} to a separate

Moved various to_str files from packet.{c,h} to a separate
to_str.{c,h}.  Resolved strange situation where ipx_addr_to_str was
declared in packet.h but defined in packet-ipx.c by moving
ipx_addr_to_str, ipxnet_to_str_punct, and ipxnet_to_str from packet-ipx.{c,h}   to to_str.{c,h}

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@3219 f5534014-38df-0310-8fa8-9805f1628bb7