fix(app-misc/ca-certificates): Never rehash certs during boot.

This kills the massive time-waster we currently have during boot to
cleanup/refresh OpenSSL's certificate directory in /etc/ssl/certs.
System provided certs are now hashed at build time and tempfiles is used
to install the proper links. The certificate bundle is also created at
build time and provided as a symlink. Since tempfiles does not help us
clean up broken links to certs that have been removed that task has been
moved to clean-ca-certificates.service which is still run on every boot.
The update-ca-certificates.service is now only responsible for
recreating the certificate bundle if required. On systems where the user
has never run update-ca-certificates the certificate bundle is just a
symlink to the copy in /usr and update-ca-certificates.service never
runs on boot. If the user does run update-ca-certificates then
update-ca-certificates.service will rewrite the bundle only if the
directory has not been modified since the last time it ran.

There are a few edge cases where if stale hash links may be left if a
system provided cert was replaced a subject that is different enough to
change the hash but not change the file name. I don't expect that to
ever happen though. Also hash collisions between the system certs and
each other or user provided certs are not handled gracefully but should
be rare. tempfiles will report wrong symlink errors in that case.
Merge pull request #681 from marineam/certs

fix(app-misc/ca-certificates): Never rehash certs during boot.