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Nginx: support kernel network stack, so we can do what fstack can't do,
e.g. unix socket, ipc (with APP on kernel network stack), packet from kernel network stack.
1. Add a new directive kernel_netw

Nginx: support kernel network stack, so we can do what fstack can't do,
e.g. unix socket, ipc (with APP on kernel network stack), packet from kernel network stack.
1. Add a new directive kernel_network_stack :
Syntax: kernel_network_stack on | off;
Default: kernel_network_stack off;
Context: http, server
This directive is available only when NGX_HAVE_FF_STACK is defined.
Determines whether server should run on kernel network stack or fstack.
2. Use a simpler and more effective solution to discriminate fstack fd(file descriptor, only socket for now) from kernel fd.

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Nginx: support kernel network stack, so we can do what fstack can't do,
e.g. unix socket, ipc (with APP on kernel network stack), packet from kernel network stack.
1. Add a new directive kernel_netw

Nginx: support kernel network stack, so we can do what fstack can't do,
e.g. unix socket, ipc (with APP on kernel network stack), packet from kernel network stack.
1. Add a new directive kernel_network_stack :
Syntax: kernel_network_stack on | off;
Default: kernel_network_stack off;
Context: http, server
This directive is available only when NGX_HAVE_FF_STACK is defined.
Determines whether server should run on kernel network stack or fstack.
2. Use a simpler and more effective solution to discriminate fstack fd(file descriptor, only socket for now) from kernel fd.

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Fix `ff_fdused_range` not work.

Simplify startup arguments and add ff_fdisused.

Changes:
1.Simplify f-stack startup arguments:"--conf, --proc-type, --proc-id".
2.add a function `ff_fdisused` to check if fd is used in f-stack.