include cleanup

The Zeek code base has very inconsistent #includes.  Many sources
included a few headers, and those headers included other headers, and
in the end, nearly everything is included everywhere, so missing
#includes were never noticed.  Another side effect was a lot of header
bloat which slows down the build.

First step to fix it: in each source file, its own header should be
included first to verify that each header's includes are correct, and
none is missing.

After adding the missing #includes, I replaced lots of #includes
inside headers with class forward declarations.  In most headers,
object pointers are never referenced, so declaring the function
prototypes with forward-declared classes is just fine.

This patch speeds up the build by 19%, because each compilation unit
gets smaller.  Here are the "time" numbers for a fresh build (with a
warm page cache but without ccache):

Before this patch:

 3144.94user 161.63system 3:02.87elapsed 1808%CPU (0avgtext+0avgdata 2168608maxresident)k
 760inputs+12008400outputs (1511major+57747204minor)pagefaults 0swaps

After this patch:

 2565.17user 141.83system 2:25.46elapsed 1860%CPU (0avgtext+0avgdata 1489076maxresident)k
 72576inputs+9130920outputs (1667major+49400430minor)pagefaults 0swaps

src/Options.cc

@@ -1,9 +1,13 @@
 // See the file "COPYING" in the main distribution directory for copyright.
 
-#include <unistd.h>
-
 #include "zeek-config.h"
 
+#include "Options.h"
+
+#include <algorithm>
+
+#include <unistd.h>
+
 #ifdef HAVE_GETOPT_H
 #include <getopt.h>
 #endif
@@ -11,8 +15,6 @@
 #include "bsd-getopt-long.h"
 #include "logging/writers/ascii/Ascii.h"
 
-#include "Options.h"
-
 void zeek::Options::filter_supervisor_options()
 	{
 	pcap_filter = {};