Move genl_ctrl_resolve out of the wifi code so it can be reused by other
interfaces based on genetlink.

https://mail.gnome.org/archives/networkmanager-list/2018-March/msg00044.html

From v4.12 the kernel appends some attributes to netlink acks
containing a textual description of the error and other fields (see
commit [1]). Parse those attributes and print the error message.

Examples:

platform-linux: netlink: recvmsg: error message from kernel: Network is unreachable (101) "Nexthop has invalid gateway" for request 12

platform-linux: netlink: recvmsg: error message from kernel: Invalid argument (22) "Local address cannot be multicast" for request 21

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2d4bc93368f5a0ddb57c8c885cdad9c9b7a10ed5

Now that we cleaned up nl_recv(), we have full control over which error
variables are returned when. We no longer need to check "errno"
directly, and we no longer need the NLE_USER_* workaround.

- adjust some coding style (space after function name).
- ensure to use g_free(), as we no longer use malloc
  but the g_malloc aliases. Nowadays, glib's malloc
  is identical to malloc from the standard library and
  so this is no issue in practice. Still it's bad
  style to mix g_malloc() with free().
- use cleanup attribute for memory handling.

Glib is not out of memory safe, meaning it always aborts the program
when an allocation fails. It is not possible to meaningfully handle
out of memory when using glib.

Replace all allocation functions for netlink message with their glib
counter part and remove the NULL checks.

With libnl3, each socket has it's own callback structure.
One would often take that callback structure, clone it, modify it
and invoke a receive operation with it.

We don't need this complexity. We got rid of all default handlers,
hence, by default all callbacks are unset.

The only callbacks that are set, are those that we specify immediately
before invoking the receive operation. Just pass the callback structure
at that point.

Also, no more ref-counting, and cloning of the callback structure. It is
so simple, just stack allocate one if you need it.

The only difference between nl_recvmsgs_report() and nl_recvmsgs() is
the return value on success. libnl3 couldn't change that for backward
compatibility reasons. We can merge them.