* process.c (retry_fork_async_signal_safe): Use vfork() if available.

  vfork() is still faster than fork() especially when the parent
  process uses big memory.

  ruby -rbenchmark -e 'a = "a" * 1_000_000_000; puts Benchmark.measure { system("true") }'
  fork:   0.000000   0.010000   0.010000 (  0.014968)
  vfork:  0.000000   0.000000   0.000000 (  0.000912)
  on Debian sid.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47363 b2dd03c8-39d4-4d8f-98ff-823fe69b080e