Previously, only KCM and MSLA implemented credential removal. Add
support for FILE (and therefore DIR), MEMORY, and KEYRING.
The FILE logic is similar Heimdal's implementation, with additional
logic for skipping removed creds during iteration. In addition to
setting endtime to 0 and changing the realm for config entries as
Heimdal does, we set authtime to -1 to make deleted entries
distinguishable from gssproxy encrypted creds and config entries.
For MEMORY, leave behind empty list elements when removing a cred will
leave behind an empty list element, in case an iterator holds a
pointer to that element.
[[hidden email]: edited commit message; made minor style and comment
changes; fixed memory leaks detected by asan]