Use nm_close() in the core to catch any improper use of close().

These static variables really never be modified.
Mark them as const, which allows the linker to mark them as
read-only.

The problem is libnl3's API, which has these parameters
not as const. Add a workaround for that. Clearly libnl3 is
not gonna modify the policy, that the API was fixed too [1]

[1] https://github.com/thom311/libnl/commit/b4802a17a7655bfeee3e9c06e649d30b96dbad3b

nm_utils_exp10() is a better name, because it reminds of the function
exp10() from <math.h> which has a similar purpose (but whose argument
is double, not gint16).

There are very few places where we actually use floating point
or #include <math.h>.

Drop that library, although we very likely still get it as indirect
dependency (e.g. on my system it is still dragged in by libsystemd.so,
libudev.so and libnl-3.so).

It can be overwritten when other arguments are evaluated.

For nl80211, we don't care about the interface name and only use it
when formatting error messages. For wext, an up-to-date interface name
should be obtained every time to minimize the chance of race
conditions when the interface is renamed.

Makes coverity happy.