messageparser: fix link recognition

The attempted fix in commit 65d3befa was based on the incorrect
assumption that md4c systematically fails to detect links that contain
at least one +, - or _. This commit removes the postprocessing step
added in 65d3befa and instead patches md4c to directly fix its link
recognition algorithm.

GitLab: #1903
GitLab: #1964
Change-Id: Ic1d6302e3250390af66611a16715a6397578abb5