selftest: Rework setting env variables for other forked binaries

selftest: Rework setting env variables for other forked binaries

Final refactor to merge the fork-and-exec code into a common function.

We can now use $daemon_ctx{ENV_VARS} to customize differences between
the forked binaries:
- samba: add in extra env variables on top of the defaults.
- dns_hub: there are no ENV variables we need to export.
- winbindd/smbd: these use the defaults, so they pass through an
undefined $daemon_ctx{ENV_VARS} (purely to make the code common across
all 5 places).

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>