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.