[syslog-ng] Use jemalloc

[Jean-Baptiste Fuzier]

I found a way for 1/ which solves the problem.

It is needed to have cap support in syslogng (--enable-caps), then
--user tells syslogng to switch to this user and --caps
cap_net_bind_service=ep allows syslogng to bind to priviledge ports
despite being not root.
LD_PRELOAD="/xxx/libjemalloc.so.2" works fine this way, as the binary
is neither setuid nor has any capabilities set.

--
Jean-Baptiste Fuzier