Restructure test_fync to use modular C so there is less duplicate code

and it can be enhanced easier.

Author: bmomjian

src/tools/fsync/README

@@ -3,7 +3,7 @@ test_fsync
 
 This program tests fsync.  The tests are described as part of the program output.
 
-	Usage:	test_fsync [-f filename] [loops]
+	Usage:	test_fsync [-f filename] [ops_per_test]
 
 test_fsync is intended to give you a reasonable idea of what the fastest
 fsync_method is on your specific system, as well as supplying diagnostic
@@ -16,5 +16,6 @@ The output filename defaults to test_fsync.out in the current directory.
 test_fsync should be run in the same filesystem as your transaction log
 directory (pg_xlog).
 
-Loops default to 2000.  Increase this to get more accurate measurements.
+Ops-per-test defaults to 2000.  Increase this to get more accurate
+measurements.