mknod: implement it

[metif12]

mknod creates special files, with -m and -Z.

Types are FIFO (p), block (b) and character (c) devices. u is rejected because Linux does not implement the POSIX "unspecified" type — measured against GNU rather than assumed.

Block and character devices need a major and minor number, packed the way makedev does. The file type is folded into the mode passed to mknod(2) alongside the permission bits, which is the detail that distinguishes a FIFO from a plain file.

The default mode is 0666 masked by the umask, as GNU applies.

How this was verified

V cannot link a Linux binary from Windows, only emit C. So I cross-compiled with -os linux, rewrote V's absolute Windows #include paths to their /mnt equivalents, and built with WSL's gcc. FIFOs turn out to be creatable unprivileged, which made real verification possible rather than only error-path checking:

• a FIFO is created with the same type and mode as GNU's (fifo 644)
• every diagnostic matches byte-for-byte, including the curly quotes GNU puts around operands (‘ff’) and the bare mknod: ex: File exists form with no cannot create and no quotes

One correction worth recording: GNU's diagnostics use U+2018/U+2019 curly quotes, not ASCII apostrophes. My first version emitted the ASCII form and looked right in the terminal, which is exactly the kind of difference that only byte-level comparison catches.

Same caveat as the other nix-only tools: the _nix_test.v suite covers only parsing and the mode arithmetic. It compiles for Linux, but I could not execute it, since V will not generate C for a test target. It will run on the ubuntu-* CI jobs.