Harden /proc/pid/mem
github-issue: #48

Introduce a new sysctl, hardening.procfs_harden, which, when enabled,
disables writing to /proc/pid/mem. Note that it is still possible to
open /proc/pid/mem with the write bit to keep some sense of
compatibility with third-party applications. Calls to write() will
fail with permission denied.
Merge remote-tracking branch 'origin/hardened/current/hardening' into hardened/current/hardening