settings: ensure config path is absolute

I have a feeling using -D would break this due to the change to cwd?
config: fix problems with config migration (fixes #2035, #2023)
webui: ensure we don't pass NULL to muxer_create for config (fixes #2017).
mpegts dvb: fix rather dubious assert (fixes #2036)

Could have ensure this was set correctly on input, but given that it was being
set if no config was passed and almost certainly it must by definition be the
same as the source mux, might as well simplify things.
url: fix stupid mistakes in regexp (fixes #2021)
timeshift: ensure no use of potentially free()'d variable (fixes #2028)