PHP 5.3/5.4: Fix loading Zend Extension PECLs

@dobbymoodge review pls. Could you run you performance tests from https://github.com/openshift/origin-server/pull/5307 as well?

```
1. Move Xdebug enabling/disabling logic to enable_modules

2. Refactor PHP enable_modules to support Zend Extensions
    
    Do not use dl() function for testing the shared libraries:
    - It can't load Zend Extensions (xdebug, opcache etc.)
    - It doesn't handle PHP Extension dependency
    - The function has been disabled from some SAPIs in upstream
    
    Add debugging and verbose messages for enable_modules logic:
        DEBUG=true ./php/bin/control restart
    
    Related:
    - Bug 1082963 - https://bugzilla.redhat.com/show_bug.cgi?id=1082963
    - Commit 02b8866be70322f4a0c0ac255479c0871e0fa4fb
```

[test] [extended:cartridge]
Obviously I meant `[[ -n "${DEBUG:-}" ]]`