Fixed minor portability issues to support FreeBSD.

Added support so sniproxy compiles in FreeBSD.

Following issues arose when compiling:
 * alloca.h doesn't exist (alloca() is defined in stdlib.h)
 * tests/binder_test.c needs to include sys/types.h since it uses funky typedefs when defining constants such as INADDR_LOOPBACK
 * autoconf/pkg-config doesn't set the HAVE_LIB{EV,UDNS,RT,PCRE} macros in src/Makefile.in so I added some minor tweaks to configure.ac.

`make check` passes for everything but reload configuration, which to my understanding is not yet supported.

If this is accepted, I will try to add a port to FreeBSD in the next versioned release.

Thanks.