fs.readFile[Sync]("/dev/stdin") does not always read the entire file

[piscisaureus]

Originally reported here: nodejs/node-v0.x-archive#7412

The root cause of the issue is that fs.readFile() stats the file and reads the amount of bytes specified in the st_size field. This works only for regular files.

The fix would be to check whether the ST_IFREG flag is set. If not, use a slower but more reliable method to read the entire file.

I think this is a "good first bug" for new contributors.

[brendanashworth]

I think this should be fixed in #1074, closing.