Before editing this code, it will be helpful to factor it out first.

Signed-off-by: charliemirabile <46761267+charliemirabile@users.noreply.github.com>

The environment variable contained hardcoded values of 3 and 4 for the file
descriptors. If more file desciptors will be shared the pipes might no longer
be file descriptors 3 and 4.

Signed-off-by: charliemirabile <46761267+charliemirabile@users.noreply.github.com>

since (an unknown amount) more values will be added to cmd.ExtraFiles, making
a dynamic array and slotting the two values in to specific indices (and then
referencing those indices in the environment variable) will make things more
flexible.

Signed-off-by: charliemirabile <46761267+charliemirabile@users.noreply.github.com>

introduce a variable to hold a count of files from systemd. this value is always zero for now,
but will come to hold the count of how many file descriptors we inherited from systemd

Signed-off-by: charliemirabile <46761267+charliemirabile@users.noreply.github.com>

…ent variable

If this environemnt variable is set and the value is an integer n, we should arrange to
have the child inherit a copy of our file descriptors 3, 4, ... 3 + n.

Signed-off-by: charliemirabile <46761267+charliemirabile@users.noreply.github.com>

…mespace

The child will inheret the file descriptors from the parent, but it needs to
pass them along to its child, the actual target process within the environment.

Signed-off-by: charliemirabile <46761267+charliemirabile@users.noreply.github.com>

This test is not currently run in CI, but it can be run locally.

Signed-off-by: charliemirabile <46761267+charliemirabile@users.noreply.github.com>