Added "bad plugin" blacklist (currently contains "view_as_html" and "folder_preferenc...

Added "bad plugin" blacklist (currently contains "view_as_html" and "folder_preferences".  Plugins known to not work, or known to be already integrated into the core (and thus should not be used as a separate plugin) should be added to this list.

Modified do_err to support the concept of fatal errors and warnings.  Fatal errors stop the script.  Warnings allow the rest of the script to continue.

do_err(string error-text, boolean fatal=true)

Minor adjustment to php version display, no alteration to actual test.
Minor adjustment to end summary (warnings, or setup looks ok text)

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@10789 7612ce4b-ef26-0410-bec9-ea0150e637f0