include <unistd.h> where necessary in files using API calls

When global <sys/mman.h> was removed from tile.h it also removed
<unistd.h> nearby.  This exposes the lack of <unistd.h> being
included where needed using APIs like close()/read()/unlink()/isatty()
the WASM build seems to show this as the header file set is structured
differently.

plot/plotVers.c

@@ -23,6 +23,7 @@ static char rcsid[] __attribute__ ((unused)) = "$Header: /usr/cvsroot/magic-8.0/
 
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
 
 /* C99 compat */
 #include <stdlib.h>