base-files: rework postinstall uci-defaults handling

base-files: rework postinstall uci-defaults handling

Some package postinstall operations, e.g. those emitted by the LuCI build
system, source and delete the uci-defaults themselves upon package insteall,
causing the generic defaults apply code to trigger shell errors like:

    .../luci-app-ddns.postinst: .: line 130: can't open './40_luci-ddns'

Rework the generic apply code to check the existence of the uci-defaults
script before trying to source it, use sed to prefilter the list of entries
from the control file and perform the directory change in a subshell,
avoiding the need for cd $OLDPWD.

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