Fixed memory leakage in col_cleanup.

Several fields in cinfo is allocated memory and must be freed here
to avoid memory leakage when recreating the packet list.

Do not allocate memory for col_expr[] as this points to static strings
or other allocated strings.

Change-Id: I840377e3c590b1a3e2a38e0537465a028041d1e9
Reviewed-on: https://code.wireshark.org/review/5137
Petri-Dish: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-by: Anders Broman <a.broman58@gmail.com>
Improved deregistering fields.

This improvement avoids use of deallocated memory (crash) if using a
deregistered field in display filter, color filter, custom column and
other cases when the field is used as "interesting field".

This functionality is currently used in http, imf and ldap preferences.

Also removed unused proto_registrar_n() as this does not work correctly
after deregistering fields.

Change-Id: I043e3bf7a98bd773c9801e712a012d1eab8a7f94
Reviewed-on: https://code.wireshark.org/review/5161
Reviewed-by: Stig Bjørlykke <stig@bjorlykke.org>
Tested-by: Stig Bjørlykke <stig@bjorlykke.org>