* ext/socket/init.c (rsock_s_recvfrom_nonblock):
  avoid arg parsing with C API
  [ruby-core:71439] [Feature #11339]
* ext/socket/basicsocket.c (bsock_recv_nonblock):
  adjust for above change, make private
* ext/socket/socket.c (sock_recvfrom_nonblock): ditto
* ext/socket/udpsocket.c (udp_recvfrom_nonblock): ditto
* ext/socket/lib/socket.rb (BasicSocket#recv_nonblock):
  new wrapper for private method, move RDoc
  (Socket#recvfrom_nonblock): ditto
  (UDPSocket#recvfrom_nonblock): ditto

Note, not adding bm_recv_nonblock.rb to benchmark/ directory
since it is non-portable.  It is only in this commit message.

Benchmark results + code
target 0: a (ruby 2.3.0dev (2015-11-12 trunk 52540) [x86_64-linux])
target 1: b (ruby 2.3.0dev (2015-11-12 avoid-kwarg-capi 52540) [x86_64-linux]

-----------------------------------------------------------
recv_nonblock

require 'socket'
nr = 1000000
msg = 'hello world'
buf = ''
size = msg.bytesize
UNIXSocket.pair(:SEQPACKET) do |a, b|
  nr.times do
    a.sendmsg(msg)
    b.recv_nonblock(size, 0, buf, exception: false)
  end
end

-----------------------------------------------------------
raw data:

[["recv_nonblock",
  [[1.83511221408844,
    1.8703329525887966,
    1.8448856547474861,
    1.859263762831688,
    1.8331583738327026],
   [1.5637447573244572,
    1.4062932096421719,
    1.4247371144592762,
    1.4108827747404575,
    1.4802536629140377]]]]

Elapsed time: 16.530452496 (sec)
-----------------------------------------------------------
benchmark results:
minimum results in each 5 measurements.
Execution time (sec)
name          a       b
recv_nonblock   1.833   1.406

Speedup ratio: compare with the result of `a' (greater is better)
name          b
recv_nonblock   1.304

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52598 b2dd03c8-39d4-4d8f-98ff-823fe69b080e