Tweak some names to better reflect what the named items are.

The old "dissector filter" code used "_filter_" in some names; the new
code used "_conv_", but they're not checking whether a *conversation* is
valid and building a *conversation* string, they're checking whether a
*filter* can be generated and building that *filter* string.

Change-Id: Iea6b1cbeb0fedefdb1fff6c23d7dee9d6e684c15
Reviewed-on: https://code.wireshark.org/review/11383
Reviewed-by: Guy Harris <guy@alum.mit.edu>