Add runtime detection for MADV_DONTNEED zeroes pages (mostly for qemu)

qemu does not support this, yet [1], and you can get very tricky assert
if you will run program with jemalloc in use under qemu:

    <jemalloc>: ../contrib/jemalloc/src/extent.c:1195: Failed assertion: "p[i] == 0"

  [1]: https://patchwork.kernel.org/patch/10576637/

Here is a simple example that shows the problem [2]:

    // Gist to check possible issues with MADV_DONTNEED
    // For example it does not supported by qemu user
    // There is a patch for this [1], but it hasn't been applied.
    //   [1]: https://lists.gnu.org/archive/html/qemu-devel/2018-08/msg05422.html

    #include <sys/mman.h>
    #include <stdio.h>
    #include <stddef.h>
    #include <assert.h>
    #include <string.h>

    int main(int argc, char **argv)
    {
        void *addr = mmap(NULL, 1<<16, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
        if (addr == MAP_FAILED) {
            perror("mmap");
            return 1;
        }
        memset(addr, 'A', 1<<16);

        if (!madvise(addr, 1<<16, MADV_DONTNEED)) {
            puts("MADV_DONTNEED does not return error. Check memory.");
            for (int i = 0; i < 1<<16; ++i) {
                assert(((unsigned char *)addr)[i] == 0);
            }
        } else {
            perror("madvise");
        }

        if (munmap(addr, 1<<16)) {
            perror("munmap");
            return 1;
        }

        return 0;
    }

  ### unpatched qemu

      $ qemu-x86_64-static /tmp/test-MADV_DONTNEED
      MADV_DONTNEED does not return error. Check memory.
      test-MADV_DONTNEED: /tmp/test-MADV_DONTNEED.c:19: main: Assertion `((unsigned char *)addr)[i] == 0' failed.
      qemu: uncaught target signal 6 (Aborted) - core dumped
      Aborted (core dumped)

  ### patched qemu (by returning ENOSYS error)

      $ qemu-x86_64 /tmp/test-MADV_DONTNEED
      madvise: Success

  ### patch for qemu to return ENOSYS

      diff --git a/linux-user/syscall.c b/linux-user/syscall.c
      index 897d20c076..5540792e0e 100644
      --- a/linux-user/syscall.c
      +++ b/linux-user/syscall.c
      @@ -11775,7 +11775,7 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
                  turns private file-backed mappings into anonymous mappings.
                  This will break MADV_DONTNEED.
                  This is a hint, so ignoring and returning success is ok.  */
      -        return 0;
      +        return ENOSYS;
       #endif
       #ifdef TARGET_NR_fcntl64
           case TARGET_NR_fcntl64:

  [2]: https://gist.github.com/azat/12ba2c825b710653ece34dba7f926ece

v2:
- review fixes
- add opt_dont_trust_madvise
v3:
- review fixes
- rename opt_dont_trust_madvise to opt_trust_madvise

src/jemalloc.c

@@ -94,6 +94,13 @@ bool	opt_junk_free =
     false
 #endif
     ;
+bool	opt_trust_madvise =
+#ifdef JEMALLOC_PURGE_MADVISE_DONTNEED_ZEROS
+    false
+#else
+    true
+#endif
+    ;
 
 zero_realloc_action_t opt_zero_realloc_action =
     zero_realloc_action_strict;
@@ -1256,6 +1263,7 @@ malloc_conf_init_helper(sc_data_t *sc_data, unsigned bin_shard_sizes[SC_NBINS],
 
 			CONF_HANDLE_BOOL(opt_abort, "abort")
 			CONF_HANDLE_BOOL(opt_abort_conf, "abort_conf")
+			CONF_HANDLE_BOOL(opt_trust_madvise, "trust_madvise")
 			if (strncmp("metadata_thp", k, klen) == 0) {
 				int i;
 				bool match = false;