add instrumentation around close() calls

In #966 what finally helped tracking down the bug was replacing all close() calls with a call to this macro

#define openvpn_close_socket(s) ({ int r; msg(D_DCO_DEBUG, "%s: openvpn_close_socket(%d)", __func__, (int)s); r = close(s); r; })

my understanding is that this is GCC specific (the bit about the int r; ... ; r; giving the macro the return value of close(s)), so it needs to be conditional.

Also, it needs a good home (not socket.h), and maybe be called openvpn_close() or such... and a decision on the right loglevel to use.

But I guess with multisockets and inotify, there might be more file descriptor hunting coming up, and making this something one can just switch on might come in handy.

Start by reviewing issue #966 and locating the close() call sites that need instrumentation. Decide how the macro should be implemented conditionally, where it belongs, what name and log level it should use, and how instrumentation can be switched on; done means the relevant close calls emit useful diagnostics without breaking supported compilers.