Set dlopen filename to null if libname is false

Fixes #99
Use fdopen to accommodate OS specific I/O streams

- STDIN_FILENO  = 0
- STDOUT_FILENO = 1
- STDERR_FILENO = 2

Fixes #101
Use darwin-specific dirent struct for OS X

Fixes #102
Log page fault measurements on linux only
Merge pull request #105 from paulfryzel/osx-tests

OS X test fixes
- Minor stylistic change