Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

include/errno.h

@@ -0,0 +1,60 @@
+// Error numbers, matching the Linux x86_64 ABI values so that musl-linked
+// programs interpret syscall failures correctly. Kernel code returns these
+// negated (e.g. `return -ENOSYS;`), exactly like Linux does internally.
+#ifndef SCHUNIX_ERRNO_H
+#define SCHUNIX_ERRNO_H
+
+#define EPERM        1   // Operation not permitted
+#define ENOENT       2   // No such file or directory
+#define ESRCH        3   // No such process
+#define EINTR        4   // Interrupted system call
+#define EIO          5   // I/O error
+#define ENXIO        6   // No such device or address
+#define E2BIG        7   // Argument list too long
+#define ENOEXEC      8   // Exec format error
+#define EBADF        9   // Bad file descriptor
+#define ECHILD      10   // No child processes
+#define EAGAIN      11   // Try again
+#define ENOMEM      12   // Out of memory
+#define EACCES      13   // Permission denied
+#define EFAULT      14   // Bad address
+#define EBUSY       16   // Device or resource busy
+#define EEXIST      17   // File exists
+#define EXDEV       18   // Cross-device link
+#define ENODEV      19   // No such device
+#define ENOTDIR     20   // Not a directory
+#define EISDIR      21   // Is a directory
+#define EINVAL      22   // Invalid argument
+#define ENFILE      23   // File table overflow
+#define EMFILE      24   // Too many open files
+#define ENOTTY      25   // Not a typewriter
+#define EFBIG       27   // File too large
+#define ENOSPC      28   // No space left on device
+#define ESPIPE      29   // Illegal seek
+#define EROFS       30   // Read-only file system
+#define EMLINK      31   // Too many links
+#define EPIPE       32   // Broken pipe
+#define ERANGE      34   // Math result not representable
+#define ENAMETOOLONG 36  // File name too long
+#define ENOSYS      38   // Function not implemented
+#define ENOTEMPTY   39   // Directory not empty
+#define ELOOP       40   // Too many symbolic links encountered
+
+// Sockets. Same Linux x86_64 values, so strerror() in a musl-linked program
+// prints the message the programmer expected rather than a neighbouring one.
+#define ENOTSOCK        88   // Socket operation on non-socket
+#define EDESTADDRREQ    89   // Destination address required
+#define EMSGSIZE        90   // Message too long
+#define EPROTOTYPE      91   // Protocol wrong type for socket
+#define ENOPROTOOPT     92   // Protocol not available
+#define EPROTONOSUPPORT 93   // Protocol not supported
+#define EOPNOTSUPP      95   // Operation not supported on transport endpoint
+#define EAFNOSUPPORT    97   // Address family not supported by protocol
+#define EADDRINUSE      98   // Address already in use
+#define EADDRNOTAVAIL   99   // Cannot assign requested address
+#define ECONNRESET     104   // Connection reset by peer
+#define EISCONN        106   // Transport endpoint is already connected
+#define ENOTCONN       107   // Transport endpoint is not connected
+#define ECONNREFUSED   111   // Connection refused
+
+#endif // SCHUNIX_ERRNO_H