Make the vmod->file_id a hash over the .vcc file.

I belive this makes our builds reproducible.

Fixes:	#2436

include/vrt.h

@@ -59,7 +59,7 @@
  *	VRT_STRANDS_string() removed from vrt.h (never implemented)
  *	VRT_Healthy() changed prototype
  *	VRT_Vmod_Init signature changed
- * 	VRT_Vmod_Fini changed to VRT_Vmod_Unload
+ *	VRT_Vmod_Fini changed to VRT_Vmod_Unload
  * 7.0 (2018-03-15)
  *	lots of stuff moved from cache.h to cache_varnishd.h
  *	   (ie: from "$Abi vrt" to "$Abi strict")

lib/libvcc/vmodtool.py

@@ -44,6 +44,7 @@
 import random
 import copy
 import json
+import hashlib
 
 AMBOILERPLATE = '''
 # Boilerplate generated by vmodtool.py - changes will be overwritten
@@ -858,6 +859,7 @@ def commit(self):
     def parse(self):
         global inputline
         a = "\n" + open(self.inputfile, "r").read()
+        self.file_id = hashlib.sha256(a).hexdigest()
         s = a.split("\n$")
         self.copyright = s.pop(0).strip()
         while s:
@@ -983,14 +985,7 @@ def vmod_data(self, fo):
         fo.write('\t.proto =\tVmod_Proto,\n')
         fo.write('\t.json =\t\tVmod_Json,\n')
         fo.write('\t.abi =\t\tVMOD_ABI_Version,\n')
-        # NB: Sort of hackish:
-        # Fill file_id with random stuff, so we can tell if
-        # VCC and VRT_Vmod_Init() dlopens the same file
-        #
-        fo.write("\t.file_id =\t\"")
-        for i in range(32):
-            fo.write("%c" % random.randint(0x40, 0x5a))
-        fo.write("\",\n")
+        fo.write("\t.file_id =\t\"%s\",\n" % self.file_id)
         fo.write("};\n")
 
     def cfile(self):