Make allowing bookmarks and multiple heads independent
Up to Heptapod 0.17, allowing bookmarks, which can be done with the following HGRC snippet:
[heptapod]
allow-bookmarks=yes
had the side effect to also allow multiple heads, hence lifting an important protection that we'd prefer to be active most of the times. As of Heptapod 0.18.0rc1, this is not true any more.
This was due to internal technical details that have become obsolete with the changes done for #351 (closed). The existing behaviour was kept back for stability reasons in the Heptapod 0.17 branch.