if $key is set to false, sqimap_login() function will use sqauth_read_password()

if $key is set to false, sqimap_login() function will use sqauth_read_password()
to retrieve user password.
sqimap_login() calls in plugins are modified to use this code.
It allows to centralize place that is used to access password information.

Older way is preserved for backwards compatibility and different IMAP login
credentials.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@11058 7612ce4b-ef26-0410-bec9-ea0150e637f0