Typedefs to structs are fine, but a typedef for a pointer seems confusing to
me. Let's avoid it.

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

- don't include "nm-default.h" in header files. Every source file must
  include as first header "nm-default.h", thus our headers get the
  default include already implicitly.

- we don't support compiling NetworkManager itself with a C++ compiler. Remove
  G_BEGIN_DECLS/G_END_DECLS from internal headers. We do however support
  users of libnm to use C++, thus they stay in public headers.

(cherry picked from commit f19aff89095ca192b8b2e37534b7a899aecd82f9)

NMSecretAgent (and in turn NMAgentManager) used the @connection argument both
for the connection data, but also for the connection path. Detangle these, and
accept the path separate from the connection.

This makes NMSecretAgent and NMAgentManager truly operate on a plain
NMConnection, without the non-obvious requirement, that the path of the
connection must be set.

This code was unused, because we never enqueued any hashes
to the @asked list. Note that hashing also might give wrong
hash collisions, so this was buggy anyway.

Also, note that impl_agent_manager_register_with_capabilities()
already ensures that duplicate agents are not registered
in the first place (find_agent_by_identifier_and_uid()).

This gives some type safety.

Have NMSecretAgent emit "disconnected" when it detects that it has
been disconnected, rather than having both the agent and the agent
manager monitor it separately.