execute: avoid logging to closed fds

Several functions called from the "sd(EXEC)" process try to log messages
when all the file descriptors are already closed, including the logging
ones. The logging functions do not expect their fds to be closed and
they hit an assertion failure. The failure wants to be logged too,
so there is an infinite recursion, ended by a SIGSEGV.

When we close all fds, we must let log.c know about it.