529563 fixes and some cleanup:

529563 fixes and some cleanup:
* require_once converted to include_once and moved to the top part of script.
* internal script functions are placed after includes
* instead of testing form vars with isset, we check sqgetGlobalVar().
* list all request uses list_addr() function instead of wide (*) search.
* listall variable is tested with isset() instead of empty().

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@9639 7612ce4b-ef26-0410-bec9-ea0150e637f0