Remove MAX_SYSCALL_NO from Syscall.xs

We generate this in syscall-hash.h
Add pmemcpy
Fix up how we send arguments to children

Use pmemcpy, and fix some faulty pointer arithmetic
Read messages from parent using fread

I wanted to use buffered reads of some sort, but I ended up reading
too much data into buffers and then discarding it.  Rather than write
my own buffered reader implementation, I'll go with C's stdlib option,
until it becomes irksome.
Remove debugging statement
Print newline after displaying a string argument