Get process information from linux /proc

[droe]

This implements -i on Linux, as per long-standing open issue #61.

Commit represents sslsplit-0.5.3-linux-pid-privsep.patch submitted by e-mail message 20180912115150.be832d62518edb080019e25b@antex.ru.

The patch author did not verify thread-safety of privsep_client* functions, need to look into this.

• [ ] In-depth code review
• [ ] Check thread-safety
• [ ] Check security of privsep server code
• [ ] Add author to AUTHORS and manual page

[garikello3d]

@droe could you please advise, why is such a useful branch not yet merged into master? I find it very handy. Thnx.