Add sys$ttyname_r and ttyname_r() + ttyname().

And print a greeting when sh starts up so we know which TTY we're on.

Kernel/Syscall.h

@@ -37,6 +37,7 @@ enum Function {
     SetMmapName = 0x2005,
     PosixReadlink = 0x2006,
     PosixWrite = 0x2007,
+    PosixTtynameR = 0x2008,
 };
 
 void initialize();