source: make indentation consistent in pgmemcache.{c,h}

* replace tabs with spaces
* use GNU style indentation for blocks
pgmemcache.c: minor code style fixes
pgmemcache: make error and warning strings more consistent
pgmemcache: remove dummy guc show functions and move variables to globals
Merge pull request #15 from saaros/master

Code refactoring with minor fixes and consistent indentation and style