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().

Kernel/Syscall.h

@@ -35,6 +35,7 @@ enum Function {
     PosixChdir = 0x2003,
     PosixUname = 0x2004,
     SetMmapName = 0x2005,
+    PosixReadlink = 0x2006,
 };
 
 void initialize();