Fix magit-insert-untracked-files & 'status.showUntrackedFiles' git config

Commit 1c4136e cleaned up magit-insert-untracked-files to always rely
rely on status.showUntrackedFiles, however it's broken when this
configuration is not set:
in such case 'git config status.showUntrackedFiles' returns nothing, not
the default value as the code expected.

Then git status was run just with "-u", which is equivalent to "-u
all". It's not what the user specified by not defining
'status.showUntrackedFiles'. It should be "-u normal" in such case.

This commit fixes and simplify this by removing the git config retrieval
altogether since git status reads this config, even with the
"--porcelain" flag.

(This git status behavior is a little ambiguous in the man: it says the
output format will be stable "regardless of user configuration", so one
can wonder why the 'status.showUntrackedFiles' user config is not
ignored. But it's about the output *format*, and not the output itself.)
magit-process-mode: don't turn on view-mode

This was a leftover from the old implementation, which didn't use a
major mode derived from `magit-mode'.  Back then `view-mode' was used so
that we could just use `view-mode-exit' instead of having to implement,
something similar ourselves.  Likewise that took care of making the
buffer read-only.

Now that we actually do use a mode derived `magit-mode' we also get that
for free.  So the use of `view-mode' no longer any benefits, and at the
same time has at least the drawback that we loose some key bindings
which are potentially useful.

Close #1259.
magit-key-mode-groups: fix two arguments
Merge pull request #1260 from Niluge-KiWi/fix-untracked-files-showUntrackedFiles-git-config

Fix magit-insert-untracked-files & 'status.showUntrackedFiles' git config
magit-anything-{,un}staged: use git-diff

In the case of `magit-anything-staged-p' that means, that we no longer
have to specify a revision, and so it now works before there is a HEAD.

Close #1256.
magit-anything-modified-p: new function

replacing `magit-everything-clean-p' whose name was even more ambiguous.