This opens any file descriptors with the O_CLOEXEC flag, which will make sure
that file descriptors won't be leaked into any child process. This was
previously an issue due to a forgotten fclose() (Yubico#136).

This adds the `e` flag to fopen() calls, making sure the `O_CLOEXEC` flag is
used. This makes sure that the file descriptor is being closed and not leaked
into child processes. This was an issues previously due to a missing fclose()
(Yubico#136).

This uses mkostemp() instead of mkstemp(), passing along the `O_CLOEXEC` flag,
which makes sure that the file descriptor is closed and won't be leaked into
any child process, which was previously an issue due to a missing fclose()
(Yubico#136).

…g fopen()

A previous commit (d51124e) added the `e` flag to the `fopen()` calls. However
this flag is not supported on all platforms (MacOS) and will be silently
dropped (see Yubico#145). This patch works around those issues by manually opening
the file descriptor using `open()` with the `O_CLOEXEC` flag, and invoking
`fd_open()` on the resulting file descriptor to open an appropriate `FILE`
stream.

This makes sure that all files used by pam_yubico will be opened with the
`O_CLOEXEC` flag on all supported platforms to mitigate issues with missing
`fclose()` invocation (see Yubico#136).