x509: Write keyUsage extension with minimal BIT STRING

x509: Write keyUsage extension with minimal BIT STRING

Avoid embedding trailing cleared bits in the BIT STRING for the
keyUsage extension.

The overwhelming majority of this changeset is correcting the
artifacts in the test suite, most of which had keyUsage with a
non-minimal encoding.  The only functional code change is in
lib/x509/x509_ext.c.

Closes: #1236
Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>