Add -o / -e options to stream output to files and prevent OOM

[spiculator]

When running mysqldump with retry I get retry process OOM killed because the dump is much bigger than RAM. I suggest adding -o and -e options to save stdout and stderr of command directly to files, truncating them every time we need to run the command again. Without these options everything should work is it does now.

[minfrin]

Went on a dig and it seems mmap and mremap can replace realloc to achieve this and have everything work the same. Let me look.

[spiculator]

I think writing directly to file should work much better than writing it first to mmaped file and then dumping to stdout where it will be redirected to another file. If user is not going to pipe it into another command then -o is the best way to do it I think.