sync, fsync, fdatasync, nice, lseek: a bunch of new calls.

* lposix.c (Psync, Pfsync, Pfdatasync): Wrap disk synchronisation
calls.
(Pnice): New call. Change process priority.
(Plseek): New call. Move current position of a file discriptor.
* specs/posix_spec.yaml (sync, fsync, fdatasync): New pending
expectations.
(nice, lseek): Specify basic behaviour of these new calls.
Submitted by zevv@github.com, from a patch by Ico Doornekamp.

Closes #93.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>