Refactoring the mmap interface. The controlling pragma is now "mmap_size"

instead of "mmap_limit".  Also change SQLITE_CONFIG_MMAP_LIMIT and
SQLITE_FCNTL_MMAP_LIMIT to SQLITE_CONFIG_MMAP_SIZE and
SQLITE_FCNTL_MMAP_SIZE, respecctively.  
The default mmap_size is now always 0, meaning that
memory mapped I/O is off by default.  There is a new compile-time option
SQLITE_MAX_MMAP_SIZE that determines a hard upper bound on the mmap_size.
Setting SQLITE_MAX_MMAP_SIZE to zero disables the memory-mapped I/O logic
and causes it to be omitted from the build.  An extra argument is added
to SQLITE_CONFIG_MMAP_SIZE that can optionally lower the SQLITE_MAX_MMAP_SIZE
at start-time. The SQLITE_MAX_MMAP_SIZE is zero for platforms where we 
know that it does not work, meaning that it cannot be turned on by mistake
on those platforms.

FossilOrigin-Name: ea1404a10abd7f68e1f8e0708c8a3199d1f79665

src/os.c

@@ -141,7 +141,7 @@ int sqlite3OsShmMap(
   return id->pMethods->xShmMap(id, iPage, pgsz, bExtend, pp);
 }
 
-#if !defined(SQLITE_DISABLE_MMAP)
+#if SQLITE_MAX_MMAP_SIZE>0
 /* The real implementation of xFetch and xUnfetch */
 int sqlite3OsFetch(sqlite3_file *id, i64 iOff, int iAmt, void **pp){
   DO_OS_MALLOC_TEST(id);