Improve file merging for mergecap and wireshark

Improve file merging for mergecap and wireshark

Refactor the file merging code by removing the duplicate logic from mergecap.c
and file.c's cf_merge_files(), into a new merge_files() function in merge.c.
Also the following user-visible changes:

 * Removed the '-T' encap type option from mergecap, as it's illogical for
   mergecap and would complicate common merge code.

 * Input files with IDBs of different name, speed, tsprecision, etc., will produce
   an output PCAPNG file with separate IDBs, even if their encap types are the same.

 * Added a '-I' IDB merge mode option for mergecap, to control how IDBs are merged.

 * Changed Wireshark's drag-and-drop merging to use PCAPNG instead of PCAP.

Bug: 8795
Bug: 7381
Change-Id: Icc30d217e093d6f40114422204afd2e332834f71
Reviewed-on: https://code.wireshark.org/review/10058
Petri-Dish: Hadriel Kaplan <hadrielk@yahoo.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Anders Broman <a.broman58@gmail.com>