Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Dtrace: improve handling of library paths.

Merge changes from illumos

906 dtrace depends_on pragma should search all library paths, not just the
current one

949 dtrace should only include the first instance of a library found on
its library path

Illumos Revisions: 13353:936a1e45726c
13354:2b2c36a81512

Reference:
https://www.illumos.org/issues/906
https://www.illumos.org/issues/949

Tested by: Fabian Keil
Obtained from: Illumos
MFC after: 3 weeks

Update illumos/dist to revision 13742:b6bbdd77139c
ZFS bits will follow in a separate commit