Cleanup c-offsets-alist for drupal and wordpress

Recent changes broke the indentation inside arglists for Drupal and
wordpress. Restoring the old situation by literally copying the old
offsets-alist is not an option for the following reasons:

 * we now inherit c-offsets-alist from the base-style (php), so we don't
   want to duplicate a different list for Drupal unless it actually needs
   completely different indentation rules.

 * old situation had bogus definitions: elements defined twice, elements
   defined to return default value or usage of `first' where the first
   defun always returns an offset.

 * old definitions contain rules not required by the standard, like 2
   spaces for case labels for wordpress. If some people need this
   behaviour we should document how to configure it, and if I'm mistaken
   it's easy to restore.

 * old definitions contain unclear rules like for K&R regions, if those
   do make sense in php-mode, I'd like to find out in what use case so
   we can test it.

Looking at the standards, there seems to be no need for
different indentation rules for drupal/wordpress/pear/psr2 so they now
all use the same c-offsets-alist.

Other changes in this commit:

 * c-offsets-alist is now alphabetically sorted
 * base style (php) now only defines elements that are actually
   different from the default

Ideas for future improvements related to coding styles:

 * there's no reason to show trailing whitespace by default for Drupal,
   and not other styles. Make this default to on or off and make it
   configurable, regardless of the used style.
 * same goes for automatic whitespace removal, this makes as much sense
   for PSR2 as it does for Drupal.
 * this also applies to the lineup-cascaded calls behaviour and to
   lesser extent hanging semicolons, no style guide specifies how to
   lineup chained methods, so it makes no sense to have it enabled for
   psr2 and (for example) disabled for pear (it's currently on for all
   styles but the user can disable it by changing the c-offsets-alist -
   which is probably hard to figure out how to do for many users)
 * remove symfony2 style (psr2 should suffice)
 * better documentation (issue #131)