uhttpd: support multiple Lua prefixes

uhttpd: support multiple Lua prefixes

Update to latest git HEAD in order to support configuring multiple
concurrent Lua prefixes in a single uhttpd instance:

  b741dec lua: support multiple Lua prefixes

Additionally rework the init script and update the default configuration
example to treat the lua_prefix option as key=value uci list, similar to
the interpreter extension mapping. Support for the old "option lua_prefix"
plus "option lua_handler" notation is still present.

Finally drop the sed postinstall hack in uhttpd-mod-lua to avoid mangling
files belonging to other packages. Since Lua prefixes have precedence
over CGI prefixes, simply register `/cgi-bin/luci` as Lua handler which
will only become active if both luci-base and uhttpd-mod-lua is installed.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>