Solaris fixes for std.stdio and std.parallelism

- Use GENERIC_IO
- Use sysconf to determine number of CPUs
(cherry picked from commit 6dcb2179e9b97e0929a97bfa70c4ab7c357df230)

Conflicts:
	std/parallelism.d
	std/stdio.d
Backport of Phobos pull request 2759.

Because of some additional changes it is not possible to cherry-pick the
commit. See https://github.com/D-Programming-Language/phobos/pull/2759.

Implements missing functionality in std.file and std.socket for Solaris.