When building with assertions, they nm_assert() for the
type. Otherwise, they are identical to a C cast.

Also, where possible, don't cast at all, but adjust
the type instead.

Also, there were a few missing casts.

This fixes the build with related bug https://bugzilla.gnome.org/show_bug.cgi?id=790697

https://mail.gnome.org/archives/networkmanager-list/2017-December/msg00005.html

The current behavior of update_unsaved is confusing. Give the argument
an enum with a name that describes better what's happening. Also, it
makes the uses grep-able.

Replace the usage of g_str_hash() with our own nm_str_hash().

GLib's g_str_hash() uses djb2 hashing function, just like we
do at the moment. The only difference is, that we use a diffrent
seed value.

Note, that we initialize the hash seed with random data (by calling
getrandom() or reading /dev/urandom). That is a change compared to
before.

This change of the hashing function and accessing the random pool
might be undesired for libnm/libnm-core. Hence, the change is not
done there as it possibly changes behavior for public API. Maybe
we should do that later though.

At this point, there isn't much of a change. This patch becomes
interesting, if we decide to use a different hashing algorithm.

In practice, this should only matter when there are multiple
header files with the same name. That is something we try
to avoid already, by giving headers a distinct name.

When building NetworkManager itself, we clearly want to use
double-quotes for including our own headers.
But we also want to do that in our public headers. For example:

  ./a.c
    #include <stdio.h>
    #include <nm-1.h>
    void main() {
        printf ("INCLUDED %s/nm-2.h\n", SYMB);
    }

  ./1/nm-1.h
    #include <nm-2.h>

  ./1/nm-2.h
    #define SYMB "1"

  ./2/nm-2.h
    #define SYMB "2"

$ cc -I./2 -I./1 ./a.c
$ ./a.out
INCLUDED 2/nm-2.h

Exceptions to this are
  - headers in "shared/nm-utils" that include <NetworkManager.h>. These
    headers are copied into projects and hence used like headers owned by
    those projects.
  - examples/C

Keep the include paths clean and separate. We use directories to group source
files together. That makes sense (I guess), but then we should use this
grouping also when including files. Thus require to #include files with their
path relative to "src/".

Also, we build various artifacts from the "src/" tree. Instead of having
individual CFLAGS for each artifact in Makefile.am, the CFLAGS should be
unified. Previously, the CFLAGS for each artifact differ and are inconsistent
in which paths they add to the search path. Fix the inconsistency by just
don't add the paths at all.