A revamp of Handbook's 'firewalls' chapter.
This is a rather lengthy patch, that attempts to fix several problems:

• Reduce repetition. There are several paragraphs in the original text that are repeated throughout the sections
• Markup fixes. Replace single quotes with <literal> tags, add <acronym>s and several other changes to improve markup consistency
• Convert to passive voice where possible, eliminate few first person references
• Several grammar and language fixes
• The original text implied that the same TCP packet would go back and forth between source and destination. Rephrase relevant paragraphs to clarify these are different packets
• Includes many fixes submitted in a PR [1] and a link fix submitted on -doc [2]
• Numerous other smaller changes, too many to mention here

PR: docs/131568 [1]
Submitted by: Chris Pepper <pepper@cbio.mskcc.org> [1]
Submitted by: Phillip Nordwall <Phillip.Nordwall@wwu.edu> [2]
Reviewed by: trhodes, keramida