Make System API independent of nix

[magicant]

As long as we re-export items from the nix crate to use in the yash_env::System API, the correctness of our semantic versioning is at the discretion of the nix crate. To put the API under our own control, we should avoid re-exporting items from the external crate.

• Errno
  • Define our own Errno #356
  • Move Errno::last and Errno::clear to System
• AtFlags
  • TBD
• FdFlag
  • TBD
• OFlag
  • TBD
• Signal
  • Migrate to our own Signal type #365
• SigSet
  • TBD
• SigmaskHow
  • TBD
• FileStat
  • TBD
• SFlag
  • TBD
• TimeSpec
  • TBD
• WaitStatus
  • TBD
• Mode
  • Eliminate nix::sys::stat::Mode from yash_env::System
  • Move yash_env::system::virtual::Mode to yash_env::system::Mode
  • Apply umask when creating a file in virtual system
  • Consider moving Mode formatting from yash_builtin::umask::format to yash_env::*