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 my opinion, this check makes more sense in core than API.  Since API comes with the expectation that it shouldn't be aware of the caller.  Core already probably does some of the same, or enables flags that can be defined in the page to change its behavior.