jailsetup: sanitize locations of utilities installed into chroot

jailsetup: sanitize locations of utilities installed into chroot

Besides Git, SSH and Girocco utilities, the only things that should
be installed into the chroot are standard "POSIX" utilities.

To that end, be paranoid about making sure the utility picked up
for installation into the chroot comes from a "standard" system
location and not some arbitrary thing in $PATH.

Also do a better job validating that the binary selected for sshd
actually exists, is executable and is readable before proceeding.

Finally, allow a custom sshd location to be configured just in case
there are some special considerations or special sshd binary needed
for the chroot.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>