timedated: be more tolerant in parsing /etc/adjtime

timedated: be more tolerant in parsing /etc/adjtime

Similarly to the previous commit, make context_write_data_local_rtc()
understand /etc/adjtime files with just one or two lines, with or without a
final newline.

Normalize the file to the current definition in hwclock(8), in the spirit of
"be liberal what you accept and strict what you produce": Add line terminators,
and set the second line to "0" if missing.

Fixes: #2638