Implement `truncate`, fix `mkfifo` on linux and add tests

• Implement truncate based upon ftruncate.
• Make sure to pass S_IFIFO when calling mkfifo on linux
• Contain the unsafe block to reading a the raw pointer in fstat
• Add tests to check for regression. Since FIFOs are not supported on Redox, don't add the mkfifo test to the permanent testsuite

merged