socket: allow explicit buffer for recv and recv_nonblock

This reduces GC overhead and makes the API more consistent
with IO#read and IO#read_nonblock.

* ext/socket/basicsocket.c (bsock_recv): document outbuf
* ext/socket/unixsocket.c (unix_recvfrom): ditto
* ext/socket/init.c (rsock_strbuf, recvfrom_locktmp): new functions
  (rsock_s_recvfrom): support destination buffer as 3rd arg
  (rsock_s_recvfrom_nonblock): ditto
* string.c (rb_str_locktmp_ensure): export for internal ext
* test/socket/test_nonblock.rb: test recv_nonblock
* test/socket/test_unix.rb: test recv
  [ruby-core:69543] [Feature #11242]

Benchmark results:

             user     system      total        real
alloc    0.130000   0.280000   0.410000 (  0.420656)
extbuf   0.100000   0.220000   0.320000 (  0.318708)

-------------------8<--------------------
require 'socket'
require 'benchmark'
nr = 100000
msg = ' ' * 16384
size = msg.bytesize
buf = ' ' * size
UNIXSocket.pair(:DGRAM) do |a, b|
  Benchmark.bmbm do |x|
    x.report('alloc') do
      nr.times do
        b.send(msg, 0)
        a.recv(size, 0)
      end
    end

    x.report('extbuf') do
      nr.times do
        b.send(msg, 0)
        a.recv(size, 0, buf)
      end
    end
  end
end

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50912 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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