Add basic symlink support.

- sys$readlink + readlink()
- Add a /proc/PID/exe symlink to the process's executable.
- Print symlink contents in ls output.
- Some work on plumbing options into VFS::open().

VirtualFileSystem/sys-errno.h

@@ -34,5 +34,5 @@
 #define EPIPE     32    // Broken pipe
 #define EDOM      33    // Math argument out of domain of func
 #define ERANGE    34    // Math result not representable
-
+#define ELOOP     40    // Too many symbolic links
 #define EOVERFLOW 75    // Value too large for defined data type