Add flag STREAM_URL_STAT_QUIET when calling urlStat.

Summary: Add flag STREAM_URL_STAT_QUIET when calling UserFile::urlStat from UserFile::stat like what it does in official implementation.

Fix vfsstream's is_file and is_dir test cases.
Closes https://github.com/facebook/hhvm/pull/4094

Reviewed By: @ptarjan

Differential Revision: D1646061

Signature: t1:1646061:1414717749:9f491a9b7bbb36d47ef7ac8c95fc48d0787500c1
Preprocessor gcc48+ performance regressions

Summary: facebook/hhvm#3808

Closes https://github.com/facebook/hhvm/pull/4065

Reviewed By: @ptarjan

Differential Revision: D1640295

Signature: t1:1640295:1414718541:0b78bc399f779276ee2130ed65af666b5edab72b