Improved Requirements and PHAR installation chapters.

- Removed always enabled PHP extensions from Requirements.
- Shortened remaining Requirements.
- Moved PHAR-specific requirements into PHAR chapter.
- Clarified extra Suhosin configuration step for using PHARs.
- #223: Restored list of extensions required by PHP core.

These extensions cannot be disabled normally, but:

> There are distributions that patch PHP to disable extensions that normally cannot be disabled.

Ensure to list them separately, so as to clarify the list of "actually" required extensions for the common scenario / average user.
Merge pull request #223 from sun/install-cleanup

Improved Requirements and PHAR installation chapters