MFV r249857:

Merge vendor bugfix for a possible deadlock related to async destroy
and improve write performance by introducing a new lock protecting
tx_open_txg.

Illumos ZFS issues:
  3642 dsl_scan_active() should not issue I/O to determine if async
       destroying is active
  3643 txg_delay should not hold the tc_lock

MFC after:	1 week
getdtablesize(2): Describe what this function actually does.

getdtablesize() returns the limit on new file descriptors; this says nothing
about existing descriptors.

MFC after:	1 week
Fix the Dvorak Programmer Keymap

PR:		conf/177572
Submitted by:	Sean DuBois <sean@siobud.com>