Fix insecure call to syslog()
Building message facility on macOS (El Cap, Xcode 7, 8) results in a
format-security warning (promoted to error by CET default flags) compiling
The attached patch resolves this, using the secure idiom recommended in the 'Notes' section of
man 3 syslog. This is tested to compile on the above platforms, though runtime cannot be tested as plugin tests are blocked by Issue #11678. Compilation and full testing has passed on CentOS6/GCC 4.9 though.