magit-with-emacsclient: use --socket-name even with default server-name

When actually using a socket for communication, then always pass the
`--socket-name' argument to the Emacsclient.  Do so even when neither
the `server-name' nor `server-socket-dir' have been set to something
different from the default values.

Just because a user did not configure Emacs to use something else,
doesn't necessarily mean that she did not attempt to so, but stopped
after making adjustments to shell init files.  I.e. there are users
with incomplete configurations, who gave up on using the Emacsclient
after having shot themselves in the foot by incompletely configuring
client/server communication.  Magit requires cannot do without the
Emacsclient and so we have to override such mistakes.
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.