OpenZFS 6739 - assumption in cv_timedwait_hires

Userland version of cv_timedwait_hires() always assumes absolute time.

Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported by: Denys Rtveliashvili <denys@rtveliashvili.name>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/6739
OpenZFS-commit: https://github.com/illumos/illumos-gate/commit/41c6413

Porting Notes:
The ported change has revealed a number of problems in the Linux-specific code,
as it was expecting incorrect return codes from pthread_* functions.
Reviewed and improved the usage of pthread_* function in lib/libzpool/kernel.c.