tools: improve explanation of 'wrap' options

tools: improve explanation of 'wrap' options

We support 'wrap' variables in /etc/frr/daemons, but the explanation
given there doesn't touch on some of the subtleties of using these
variables.

The variables were designed for use with Valgrind, which has special
behavior when run with programs that daemonize; Valgrind will intercept
the fork()'d child process and run itself instead of the child. This
behavior allows it to follow the same forking semantics as the target
program.

For virtually every other wrapper, the wrap variables do not work as
demonstrated because the wrapper programs do not daemonize. If the
wrappers do not daemonize, they will block the init script. The examples
given with "perf" for example simply do not work, because perf remains
in the foreground even as it tracks forked children.

This patch adds an explanation of the behavior expected by the init
script and offers a solution for getting that behavior.

Signed-off-by: Quentin Young <qlyoung@nvidia.com>