ply-kmsg-reader: Fix doubly underlined text output

ply-kmsg-reader: Fix doubly underlined text output

Currently plymouth applies Select Graphic Rendition (SGR) attribute 21
to disable bold text. However, per ECMA-48 [1], this attribute is for
doubly underlined text, not for disabling bold, resulting unintended
doubly underlined output on boot. Fix it by conditionally applying
standard bold attribute (SGR 1) on bold text.

[1] 8.3.117, https://www.ecma-international.org/wp-content/uploads/ECMA-48_5th_edition_june_1991.pdf

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>