Signal parent once child takes over

[mechanicker]

Fix for: https://github.com/kahing/goofys/issues/230

Parent process was blocked waiting on wg sync.WaitGroup preventing autofs from managing the forked daemon child process. Sending the signal to parent once child is ready ensures parent process exits and lets child process handle mounting and serving FUSE requests.

Tested this in an EC2 instance with existing autofs (for NFS) with goofys:

$ cat /etc/auto.master
/mnt/data  /etc/auto.nfs
/mnt/data/s3  /etc/auto.s3

$ cat /etc/auto.s3
data -fstype=fuse,_netdev,allow_other,--region=us-west-2 :goofys#s3-bucket-name

# List contents of git repository in S3 bucket
$ ls /mnt/data/s3/data/repos/puntar/
HEAD  config  description  hooks  info  objects  packed-refs

[mechanicker]

@kahing Could you please help review and merge this important fix to get goofys working with automounter/autofs?