Use posix_spawn instead of fork+exec

Use posix_spawn instead of fork+exec

Client code might allocate a lot of memory before starting the mount.
Fork is slow for processes that are using a lot of memory.  But
posix_spawn fixes that.

Another issue with fork is if the process is also doing RDMA - this
might lead to data corruption, as least if memory used for RDMA
is not marked with MADV_DONTFORK.  At least with linux kernels
before 5.12.
Also see https://blog.nelhage.com/post/a-cursed-bug/ for more details

Change by Bernd:
This also prepares the new fusermount option "--comm-fd", but keeps
the previous way to pass the parameter as env variable. In a future
release (exact data to be determined) we are going to remove usage
of the env variable and will switch to the new parameter.