Add quotes.

From Lasse Collin.
Simplify locking in logger.sh.
Simplify locking in the main backup script.

Now it also avoids writing to the log about locking
if the lock can be immediately acquired.
Tidy up the finding of the previous backup.

Some repeated code was put into a separate function.

The path to --link-dest is now a relative path.

The old version used sort -n but since -n only
does the first field in numeric order, it using -n
was useless. The snapshot names sort properly
when using normal alphabetical sort so -n didn't
do any harm even if it didn't do any good either.
Get rid of logger.sh.

The functionality of logger.sh was integrated into
the log function in functions.sh.

The log function no longer uses a temporary file.
Move get_prev_backup to functions.sh and use it there too.

Also fix the use of "local foo=bar".