Solved recursion problem and increased efficiency of template_construct hook by addin...

Solved recursion problem and increased efficiency of template_construct hook by adding file name to the end of the hook name, such as template_construct_login.tpl.  Changed hook type to use the newly reworked concat_hook_function which can support merged arrays retued from the plugins.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@11735 7612ce4b-ef26-0410-bec9-ea0150e637f0