lib: noisy deprecation of child_process customFds

Use a util.deprecate wrapper to issue warnings like any other
deprecated API. The option has been marked as deprecated in the docs
since v0.5.11.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
doc: remove customFds mentions

The feature has been marked as deprecated since v0.5.11.

Signed-off-by: Fedor Indutny <fedor@indutny.com>
test: Remove references to customFds

Option has been deprecated since v0.5.11

Signed-off-by: Fedor Indutny <fedor@indutny.com>