Fix the default colors to use 39, the default foreground color

[taoky]

This reverts commit 07b18a3, and fix #2903.

According to Linux manpage console_codes(4), "color 39" is not a undefined value:

39         set default foreground color (before Linux 3.16: set underscore off, set default foreground color)

I'm afraid that what in #2032 does not hold, and setting its value to 37 hurts users with light background, as "color 37" is:

37         set white foreground

[taoky]

And according to ECMA standard:

https://www.ecma-international.org/wp-content/uploads/ECMA-48_2nd_edition_august_1979.pdf, page 41 (PDF page 49):

39 (reserved for future standardization)

https://www.ecma-international.org/wp-content/uploads/ECMA-48_3rd_edition_march_1984.pdf, page 46 (PDF page 53), and https://www.ecma-international.org/wp-content/uploads/ECMA-48_5th_edition_june_1991.pdf, page 62 (PDF page 76):

39 default display colour (implementation-defined)

[nicowilliams]

Wouldn't we also need to update the main manual?

Otherwise LGTM.