As this is the only moderately critical fopened for writing file,
create an alternate path to use open and fdopen for non-glibc platforms
that support O_CLOEXEC (BSDs).

Tested on Linux (by modifing the GLIBC defination) to take this
alternate path:

$ cd /proc/23874
$ more fdinfo/71
pos:    0
flags:  02100001
mnt_id: 24
$ ls -la fd/71
l-wx------. 1 dan dan 64 Mar 14 13:30 fd/71 -> /dev/shm/var_auto_i7rl/mysqld.1/data/ib_buffer_pool.incomplete