tree-wide: invoke rlimit_nofile_safe() before various exec{v,ve,l}() invocations

Whenever we invoke external, foreign code from code that has
RLIMIT_NOFILE's soft limit bumped to high values, revert it to 1024
first. This is a safety precaution for compatibility with programs using
select() which cannot operate with fds > 1024.

This commit adds the call to rlimit_nofile_safe() to all invocations of
exec{v,ve,l}() and friends that either are in code that we know runs
with RLIMIT_NOFILE bumped up (which is PID 1 and all journal code for
starters) or that is part of shared code that might end up there.

The calls are placed as early as we can in processes invoking a flavour
of execve(), but after the last time we do fd manipulations, so that we
can still take benefit of the high fd limits for that.

src/basic/process-util.c

@@ -1521,6 +1521,8 @@ int fork_agent(const char *name, const int except[], size_t n_except, pid_t *ret
                 safe_close_above_stdio(fd);
         }
 
+        (void) rlimit_nofile_safe();
+
         /* Count arguments */
         va_start(ap, path);
         for (n = 0; va_arg(ap, char*); n++)