Remove MONGOOSE_NO_DAV on DAV_AUTH_FILE options

Building with -DMONGOOSE_NO_DAV will fail.
Tyop fix.
Merge pull request #351 from da4089/master

Tyop fix.
Merge pull request #350 from solomonhuang/nodav

Remove MONGOOSE_NO_DAV on DAV_AUTH_FILE options
updated faw and tutorials
I think that the socket is invalid, the return should be -1.
Merge pull request #352 from ranzhengyuan/some-idea

I think that when the socket invalid, the return value should be -1.
Merge pull request #1 from cesanta/master

merge