Improve current timezone initialization, default to UTC

This is what I hope will be a better fix for issue http://www.mantisbt.org/bugs/view.php?id=17747 (compare with @vboctor's original fix in PR #380).

In addition to fixing the original problem (i.e. PHP warnings when `date.timezone` is not set on PHP >= 5.4), this PR includes the following changes and improvements:
- move timezone setting code to a new API function
- Timezone selection list does not include 'UTC'  (http://www.mantisbt.org/bugs/view.php?id=17751)
- remove RHEL/CentOS-specific check (no longer needed as it only applies to PHP 5.1 and we require 5.3)
- fix detection of admin check scripts and improve the check itself
- revised documentation
In other words, if I understand you correctly you would prefer to see something like this ?

- in core.php 

  ``` 
date_set_current_timezone( basename( dirname( $_SERVER['SCRIPT_NAME'] ) ) != 'check' );
```
- in core/date_api.php

  ``` php
function date_set_current_timezone( $p_set_config = true ) {
	[...]
		# initialize the default timezone when executing admin checks
		if( $p_set_config ) {
	[...]
```

As mentioned before I don't really see the benefit, but if you think that's how it should be done I don't mind making the change.