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
That was my initial approach, but I changed it because during tests I realized that we always need to set the global default_timezone (i.e. not just when user pref is not set or invalid), so I thought it made more sense that way.