Add `-e` option to not validate empty password.

When the XF86ScreenSaver key is used to put a laptop to sleep (or to
trigger the screensaver), the key may "bounce" on resume. This is
annoying as i3lock will try to validate several empty passwords and
wait several seconds before accepting a legit password.

Some users may want to validate an empty password: PAM may rely on
other sources to unlock the screen, like the presence of a token or
the proximity of some Bluetooth device. Hence, we don't forbid this
possibility and provide an command-line option for users not willing
to validate empty passwords.
Add -e to --help
Bugfix: update the initial keyboard modifier state (Thanks lee, Ran)

This should make the numpad work correctly.
Small improvements to manual page.

Use a bulleted list for the IMPROVEMENTS section. Fix some
grammar. Remove the use of "fuck" which may offend some people.
Run authentification in different process

When having i3bar in “hide” mode, it could previously be brought up
while the authentication was running. This is not 100% fixed with this
change, but a lot better, as the normal “raise i3lock to the top”
behavior is not blocked now anymore.

fixes #895
Bug fix: Call clear_input() when the password is wrong.

clear_input() calls start_clear_indicator_timeout() to clear the
indicator after 1 second. This had as a result the screen to be cleared
when the "verifying…" message was showed, so the "wrong!" message didn't
show up when the password was wrong.

First bad commit: a305e62
Revert "Bug fix: Call clear_input() when the password is wrong."

This reverts commit 33d953fbdf4f8630e919643974f32ce6793c7c2b,
in preparation for reverting commit a305e62
Revert "Run authentification in different process"

This reverts commit a305e622a6cad3f9ba267d6810fee54b7c7c86c3.

In the next commit, the same functionality will be achieved in a cleaner
way.
Re-raise i3lock when obscured in a separate process

This is cleaner than commit a305e62 (running authentication in a
separate process) because we don’t lose the feature that X11 will buffer
KeyPresses for us while the process blocks on PAM. See
http://cr.i3wm.org/patch/305 for the amount of code (and new known bugs)
that would have to be introduced to make a305e62 work.
Turn on the screen on successful authentication

As described in ticket #1114, the screen may be left turned off on successful
authentication. This commit fixes this behaviour by turning the screen back on
after the authentication.

Fixes #1114
Merge branch 'master' of git://code.i3wm.org/i3lock