Darwin read()/pread()/fread() and write()/pwrite()/fwrite() calls can only...

Darwin read()/pread()/fread() and write()/pwrite()/fwrite() calls can only write INT_MAX bytes at a time.

Modify all read/write calls to allow any system to define MAX_READ_SIZE or MAX_WRITE_SIZE so large reads and writes can be divided up correctly.

<rdar://problem/20471411>

llvm-svn: 234455