Yikes - saving of Default Settings must have been broken for some

Yikes - saving of Default Settings must have been broken for some
time! The load code in settings.c was translating an empty string
into "Default Settings" to pass to {win,ux}store, whereas the save
code was passing the empty string straight down and expecting it to
be dealt with. So, a policy decision: the precise name of the
"Default Settings" special session _at the storage level_ is up to
the individual platform storage code to decide, and all platforms
MUST assume Default Settings is meant if they receive NULL or the
empty string as a session name.

git-svn-id: svn://svn.tartarus.org/sgt/putty@2974 cda61777-01e9-0310-a592-d414129be87e