Add support for rlimits

[MarkusBauer]

This PR adds resource constraints to the child process, using setrlimit.

Example usage:

use nix::libc::{RLIMIT_AS, RLIMIT_CPU, RLIMIT_NICE, RLIMIT_NOFILE, RLIMIT_NPROC, RLIMIT_RSS};

cmd.set_rlimit(RLIMIT_NOFILE, 10);
cmd.set_rlimit(RLIMIT_CPU, 1);
cmd.set_rlimit(RLIMIT_NPROC, 3);
cmd.set_rlimit(RLIMIT_AS, 64 * 1024 * 1024);
cmd.set_rlimit(RLIMIT_RSS, 64 * 1024 * 1024);
cmd.set_rlimit(RLIMIT_NICE, 0);

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Availability: If anyone wants to use this and other features before it's merged, check out my temporary fork. It includes #23 #27 #29 #33 #34 #35 and #36 with all conflicts resolved.
Add to your Cargo.toml: unshare = { git = "https://github.com/MarkusBauer/unshare.git" }