$current_user unfounded assumptions

[filosofo]

Currently, a number of places in the code (mainly in MS areas) assume that $current_user is a defined global object, when in fact that's not a safe assumption to make.

For most of the cases, $current_user is brought into scope just to get the current user's ID; in that case get_current_user_id() is much more reliable.

For the others, $current_user should be explicitly defined to make sure that

• If it has not been instantiated, it now will be.
• The pluggable functions get to instantiate it, if applicable.

Patch calls get_current_user_id() when that's what's desired; in the other instances it makes sure that it is defined appropriately.

[ryan]

(In [15315]) Use get_current_user() and get_current_user_id() instead of global . Props filofo. see #13934 for trunk

[ryan]

(In [15331]) Don't perform current user ops in switch_to_blog() or restore_current_blog() if init hasn't fired. Avoids wp_get_current_user() not defined error when those functions are run before init. Ops on current user prior to init are meaningless anyway. see #13934

[ryan]

(In [15332]) Fix inverted logic in [15331]. see #13934

[ryan]

(In [15452]) Use get_current_user() and get_current_user_id() instead of global current_user object. Props filofo. fixes #13934 for 3.0.1