log: add %p option to --logfile, expands to pid

Review URL: https://chromiumcodereview.appspot.com/12093008
Patch from Ben Noordhuis <info@bnoordhuis.nl>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13868 ce2b1a6d-e550-0410-aec6-3dcde31c8c00