Fix some warnings - declare some arguments, variables, and structure

Fix some warnings - declare some arguments, variables, and structure
members to be const pointers when that's possible, and throw in some
casts when the GLib API fails to have properly consted arguments.

Use ep_strdup_printf() in some cases.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@25596 f5534014-38df-0310-8fa8-9805f1628bb7