iopl.2: Updating description of permissions and disabling interrupts

iopl.2: Updating description of permissions and disabling interrupts

Update description of permissions for port-mapped I/O set
per-thread and not per-process. Mention that iopl() can not
disable interrupts since Linux 5.5 anymore and is in general
deprecated and only provided for legacy X servers.

See https://bugzilla.kernel.org/show_bug.cgi?id=205317

Reported-by: victorm007@yahoo.com
Signed-off-by: Thomas Piekarski <t.piekarski@deloquencia.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>