refactor: use cfg-aliases for feature flags

[joshka]

Introduces aliases for each feature flag as well as for the main combinations of http version and client/server roles. This makes it significantly easier to understand which feature flags are required on any given line as the aliases require less parsing of nested cfg expressions.

• Drops 122 LoC.
• Each cfg attribute is about half the width
• No more complex any() within all() conditions