Fix oops in r1.30.
Wrap iconv.h also with #ifdef __NetBSD__.
Remove unused extended attributes kernel options

As Masao Uebayashi pointed to me, UFS_EXTATTR_AUTOSTART, LFS_EXTATTR_AUTOSTART
and UFS_EXTATTR_AUTOCREATE are not used anywhere in the code. Remove them
as they have been obsolete for a long time:
UFS_EXTATTR_AUTOSTART was replaced by mount -o extattr
LFS_EXTATTR_AUTOSTART was created to match obsolete UFS_EXTATTR_AUTOSTART
UFS_EXTATTR_AUTOCREATE was replaced by sysctl vfs.ffs.extattr_autocreate
Deal with the potential pressence of long double support.
Replace callout_stop with callout_halt

In order to call callout_destroy for a callout safely, we have to ensure
the function of the callout is not running and pending. To do so, we should
use callout_halt, not callout_stop.

Discussed with martin@ and riastradh@.