add instrumentation around close() calls

[cron2]

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.