normalize filenames of maintenance scripts

Use lowerCamelCase.php format for all files (per [[mw:CC#File_naming]]),
and make filenames more specific:
- clear_stats.php -> clearCacheStats.php
- clear_interwiki_cache.php -> clearInterwikiCache.php
- initStats.php -> initSiteStats.php
- proxy_check.php -> proxyCheck.php
- stats.php -> showCacheStats.php
- showStats.php -> showSiteStats.php

Also changed the class names accordingly (per [[mw:CC/PHP#Naming]]),
and make class names more specific:
- clear_stats -> ClearCacheStats
- InitStats -> InitSiteStats
- CacheStats -> ShowCacheStats
- ShowStats -> ShowSiteStats

Updated files that made references to the changed files/classes:
- DefaultSettings.php and SpecialBlockme.php (proxy_check.php -> proxyCheck.php)
- maintenance/showSiteStats.php (initStats.php -> initSiteStats.php)
- maintenance/README and docs/memcached.txt (stats.php -> showCacheStats.php)
- docs/maintenance.txt and docs/memcached.txt (clear_stats.php -> clearCacheStats.php)

Thanks Hashar for the initial help and encouragement :)

Change-Id: I60f76fc971e06e1b710dcda35f9c2d931b93bdd7
Merge "normalize filenames of maintenance scripts"