Fix disabling of bold and fastblink

According to ECMA-48¹ 8.3.117, an attribute value of 21 is "doubly
underlined", while 22 is "normal colour or normal intensity (neither
bold nor faint)".

Additionally, 25 is "steady (not blinking)", which likely means neither
slow blink nor fast blink.

¹: http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-048.pdf

Signed-off-by: Roberto E. Vargas Caballero <k0ga@shike2.com>
Remove difference between fast and slow blinking

One blinking mode is good enough, and two is too much. The best aproach
is emulate the fast blinking with the slow blinking, that it is more
used.
It is removed the flag ATTR_FASTBLINK because it has not a different
meaning of ATTR_BLINK, so it is not needed.

Signed-off-by: Roberto E. Vargas Caballero <k0ga@shike2.com>
Merge branch 'master' of http://git.suckless.org/st