Instantiate plugins AFTER connecting to the db.

This allows plugins to use the connection from their constructor if the
script created it, dpeending on $_no_db_connection global variable.
This revert commit b43084e022695d6b506799a1815fb4735bd57d13. I did a few
tests, mainly on intro page, but couldn't find the bug this commit tried
to fix...
plugin report: Remove non used global var in constructor.
Add try/catch blocks around plugin instanciation so they can throw an exception if needed.

This is usefull if a plugin don't want to start for some reasons.
Throwing an exception from the plugin constructors allows to remove this
plugin entirely during the script execution.
Instanciate $plugin_manager from login.php script if needed

This answers the question in commit message:
7d89d7cca40b5a8c3ac1874436630993f518410c. When the script require a
database connexion and the user must login, or in some cases of some
other related errors, we ar eincluding the login.php script and exit
immediatly. The login.php script requires the $plugin_manager, but it
is instanciated AFTER the connection codeblock (see commit 7d89d7cc),
leading to a FATAL error.
fixed a minor bug of Chinese UTF8 translation
change chinese translation of "null" and "empty"
Review/Update BUGS CREDITS HISTORY and TODO before release
Mark PPA release 5.1
Update/Fix History
Fix typo in variable name

It only concerns an fatal error message, no bad impacts
Cosmetic fix on Vacuum confirmation page for the "Freeze" checkbox

All other options checkbox have a space between the checkbox and their
label.
Completely udpate chinese utf8 according to english.
langcheck will give correct instruciton if the language parameter is added with php extension
Rename PPA version from 5.1 to 5.2-dev.
Explicit support for PG 9.3.
Processes: Fix <= 9.1 to show only db related processes & clean dbname.

Fix bugs introduced in commit d571ecae7b by ioguix on 2012-09-21.

I can't tell why these changes crept in along with the rest of
the fixes in that commit.  It looks like a testing version was
committed.
Fix so signals are sent to the right pid.
Add support for user-sent signals. (9.2+)
Make process screen report information in a way that makes sense in postgres 9.2
Also add in the waiting option, making it work back to at least 8.1
doTree() does not take an argument.