${MAINPID} is considered a variable reference by cmake and
would be substituted to empty when installing. Instead of escaping
the dollar sign, simply use $MAINPID instead which does the same
thing in this case and is more concise.
Using @CMAKE_INSTALL_FULL_BINDIR@ breaks when kill(1) is not in the
same directory as the installed binaries, which is very likely when
the cmake installation prefix is e.g. /usr/local. Skip hardcoding
the path to kill(1) and allow systemd to do a PATH lookup instead.