Periodic maintenance PR

[elicn]

Yet another maintenance PR including bug fixes and various improvements around security and robustness.

Highlights:

• Revamped the fs_mapper module (might break a few old DOS examples)
• Eliminated potential path traversal risks where emulated program could access arbitrary resources on the host file system
• Patched many POSIX system calls to make them better compliant to spec
  • fcntl
  • unistd
  • stat (partially)
• Replaced the dummy POSIX shm syscalls implementation with a more compliant one
• Added an initial implementation for POSIX ipc syscall
• Fixed a bug in which the emulated program could close host standard streams
• As always, opportunistic PEP8-friendly tweaks to keep it clean an tidy