Clean up CreateFile loop in ReadUTF8File

[DHowett]

ReadUTF8File in the Settings library tries three times to read the settings file. If the size changes while it is being read, it'll try again.

Right now, the code looks like this (roughly):

while(tries) {
    open
    check permissions
    read in full
    check read size
}

It's expensive to open the file and check the permissions every time. Instead, we could:

open
check permissions
while (tries) {
    reset file pointer
    read in full
    check read size
}

That'll save us closing/reopening the file up to three times.

[DHowett]

Though -- what if we fail to open the file because somebody has it locked? Wouldn't we want to retry some time later so as to not frag the user's settings as "invalid"? That suggests that we may want to retry opening it as well...