certctl: Reimplement in C

Notable changes include:

* We no longer forget manually untrusted certificates when rehashing.

* Rehash will now scan the existing directory and progressively replace
  its contents with those of the new trust store.  The trust store as a
  whole is not replaced atomically, but each file within it is.

* We no longer attempt to link to the original files, but we don't copy
  them either.  Instead, we write each certificate out in its minimal
  form.

* We now generate a trust bundle in addition to the hashed diretory.
  This also contains only the minimal DER form of each certificate.
  This allows e.g. Unbound to preload the bundle before chrooting.

* The C version is approximately two orders of magnitude faster than the
  sh version, with rehash taking ~100 ms vs ~5-25 s depending on whether
  ca_root_nss is installed.

* We now also have tests.

Reviewed by:    kevans, markj
Differential Revision:  https://reviews.freebsd.org/D42320
Differential Revision:	https://reviews.freebsd.org/D51896

etc/mtree/BSD.tests.dist

@@ -1255,6 +1255,8 @@
         ..
     ..
     usr.sbin
+        certctl
+        ..
         chown
         ..
         ctladm