libroot: check timeout interval parameter in ppoll(), pselect() and select()

* also introduce timeval_to_bigtime
* time_private is c++ only

Change-Id: I2bddbe5f95240735c7b42ccf429dcbb3f1e8bb1b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5220
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>