linum-mode plays more nicely with other margin-setting extensions

linum-mode plays more nicely with other margin-setting extensions

linum.el will only modify the left margin if it needs to, and will
only reset the it back to 0 if it guesses that no-one has touched that
margin in the meantime.

As such, this is a more of a workaround than an actual fix, but fixes
the problems described in bug#20674 regarding the interaction with
modes such as darkroom-mode and olivetti-mode.

A similar fix was commited to nlinum.el in ELPA.git's
e7f5f549fbfb740b911fb7f33b42381ecece56d8

* linum.el (linum-delete-overlays): Restore margins more
criteriously.
(linum-update-window): Set margins more criteriously.