sh: Add test for nested alias.
dtrace: remove unexplained 16MB limitation from dt_alloc/dt_zalloc

The limitation was introduced in r178556 without any note or comment.
It seems pretty artificial and now it leads to problems like the following:
  $ dtrace -x bufsize=17m -n ...
  dtrace: processing aborted: Memory allocation failure
OpenSolaris and illumos never had this limitation.

Sponsored by:	HybridCluster
Hide now harmless warning from dmesg.

MFC after:	1 week