BUG/MEDIUM: systemd-wrapper: fix locating of haproxy binary

BUG/MEDIUM: systemd-wrapper: fix locating of haproxy binary

OpenBSD complains this way due to strncat() :

src/haproxy-systemd-wrapper.o(.text+0xd5): In function `spawn_haproxy':
src/haproxy-systemd-wrapper.c:33: warning: strcat() is almost always misused, please use strlcat()

In fact, the code before strncat() here is wrong, because it may
dereference a NULL if /proc/self/exe is not readable. So fix it
and get rid of strncat() at the same time.

No backport is needed.