build: add --enable-more-asserts configure option and nm_assert() macro

build: add --enable-more-asserts configure option and nm_assert() macro

NM already has two kinds of assertions:
 - g_assert*(), conditionally compiled via #ifndef G_DISABLE_ASSERT
 - g_return*(), conditionally compiled via #ifndef G_DISABLE_CHECKS

In theory, one should be able to disable both asserts and NM should
still work correctly (and possibly more efficient). In practice,
hardly anybody is testing such a configuration and it might be broken.
Especially, we don't disable asserts for production builds, both because
of less test coverage and because it might reduce our ability to debug.

Add a new configure option --enable-more-asserts, which defines
NM_MORE_ASSERTS and nm_assert(). This is for expensive asserts,
that -- contrary to the asserts above -- are disabled by default.
This is useful for extended debugging.