Fix detection of point to insert signatures on a pending consensus.

We were looking for the first instance of "directory-signature "
when instead the correct behavior is to look for the first instance
of "directory-signature " at the start of a line.

Unfortunately, this can be exploited as to crash authorities while
they're voting.

Fixes #40316; bugfix on 0.2.2.4-alpha.  This is TROVE-2021-002,
also tracked as CVE-2021-28090.