Fix some issues in lz4 support

[john-cado]

Ran into some issues with lz4 support where we attempt to reopen the file for writing as a zip later in the process. Suggest reviewing commits separately since this includes some initial tidying up:

• Move fields from LinuxSystem to constants/locals, making LinuxSystem and WindowsSystem __init__s identical
• Dedup init methods
• Open output file once and have all methods write to it instead of repeatedly opening for appending