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]
This is 4 `exec()` in a row, in a loop. I haven't done a speed test yet, but I think this will slow down the script. I think some of this can be replaced with bash builtins, but it would probably be more effective to defer these changes until after the loop, and then do them all in one go if possible.