libc/iconv: Fix race condition with setting FD_CLOEXEC.
libc/iconv: Fix race condition with setting FD_CLOEXEC.

A fork/exec could happen between open and fcntl, leaking a file descriptor.
Using O_CLOEXEC fixes this and as a side effect simplifies the code.

NetBSD already had this (I checked this after making the change myself).

Reviewed by:	gabor

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