BUILD: checks: remove the last strcat and eliminate a warning on OpenBSD

BUILD: checks: remove the last strcat and eliminate a warning on OpenBSD

OpenBSD emits warnings on usages of strcpy, strcat and sprintf (and
probably a few others). Here we have a single such warning in all the code
reintroduced by commit 0ba0e4a ("MEDIUM: Support sending email alerts") in
1.6-dev1. Let's get rid of it, the open-coding of strcat is as small as its
usage and the the result is even more efficient.

This fix needs to be backported to 1.6.