sysctl: Check path is under /proc/sys

sysctl: Check path is under /proc/sys

sysctl would try to read/write any path you gave it either on
the command line or configuration file. It would append /proc/sys
on the path but not check for any sneaky path traversal with ../

This commit means it first resolves all paths using realpath(3)
and then checks the path starts with "/proc/sys/"

At first I thought this might be a non-issue, but perhaps someone
could put a file into the sysctl configuration path and.. do
something? Anyway its a 8-line fix and makes things more correct.

References:
 #179

Signed-off-by: Craig Small <csmall@dropbear.xyz>