lib: intro `user_exists` and `group_exists`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/privileges: prevent seg faults with asserts and require in doc

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/curl: use more precise return types

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>