Many of Daniel's fixes.

I'm returning the number of bytes actually copied so that the client has some
warning when it reads past the end of the buffer.

I'm keeping the distinction between getByte() and getBytes() for now for
subclasses that use functions like ptrace() on Linux and only have a restricted
interface.  This makes their implementation easier, and subclasses can always
write a one-line implementation of readByte() that uses their custom 
readBytes().

llvm-svn: 77225