system-source: A tool to expand system()

When system() was turned into a C module, it became harder to figure out
what it expands to. This patch adds a small and simple tool that will
create a temporary syslog-ng config file, and extract the part system()
expanded to. This fixes #94.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #134 from algernon/f/system-expander

system-source: A tool to expand system()