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.

• 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.

• The DISTBASE concept has been dropped; the same effect can be achieved by adjusting DESTDIR.

• We now also have rudimentary tests.

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