MFC r259113, r259115, r259144, r259148:

r259113 (dteske):
  Fix failed attempt to send pkg(8) stderr to /dev/null

r259115 (dteske):
  Prevent truncating /tmp/bsdinstall_log each time we
  exec a module.

r259144 (dteske):
  Fix a regression after successfully installing to encrypted
  ZFS root, the passphrase is not accepted and a message about
  "incorrect key" is displayed.

r259148 (dteske):
  Fix a regression resulting in mountroot prompt after attempting
  to install to encrypted ZFS root (caused by a typo in a
  variable name -- ZFSBOOT_BOOT_FSNAME -> ZFSBOOT_BOOTFS_NAME).

Sponsored by:	The FreeBSD Foundation