luci-base: properly handle repeated POST parameters

Restore the old luci.http behaviour of converting repeated POST params into
single tables holding all values instead of letting each repeated parameter
overwrite the value of the preceeding one.

Fixes, among other things, the handling of CBI dynamic list values.

Fixes #1752
Fixes 59dea023

 ("luci-base: switch to lucihttp based POST data processing")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>