Added PledgePromises and PledgeExecpromises functions
Added the pledgeAvailable helper function to check for pledge(2)
availability

Change-Id: Ib40e7eb481f70e4c02219db0ad445dfaa92aba2b
Reviewed-on: https://go-review.googlesource.com/c/142898Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>