Resolve "Identify unused source files via gcov CI job"

The gcov CI job identified several "unused" source files:

File                                       Lines    Exec  Cover
bin/confgen/unix/os.c                          6       0     0%
bin/dnssec/dnssec-keyfromlabel.c             411       0     0%
bin/named/fuzz.c                               2       0     0%
lib/dns/dbtable.c                            102       0     0%
lib/dns/lookup.c                             226       0     0%
lib/dns/mb_7.c                                94       0     0%
lib/dns/md_3.c                                95       0     0%
lib/dns/mf_4.c                                95       0     0%
lib/dns/portlist.c                           122       0     0%
lib/dns/tsec.c                                76       0     0%
lib/isc/backtrace.c                           10       0     0%
lib/isc/bufferlist.c                          18       0     0%
lib/isc/pk11.c                               559       0     0%
lib/isc/unix/pk11_api.c                      315       0     0%
lib/isc/unix/syslog.c                          8       0     0%
lib/ns/lib.c                                  19       0     0%

Some seems legitimately unused files (dbtable.c, portlist.c, bufferlist.c), but most are not: excluded because of configure options used (fuzz.c, pk11*.c, dnssec-keyfromlabel.c), or having too specific content (backtrace.c, syslog.c, lib.c). Some, e.g. lookup.c looked a bit abandoned but I was unable to rip them out.

Closes #2060 (closed)