Support the GNU-specific syntax ".IP \\[bu]" for bullet lists in man(7)

Support the GNU-specific syntax ".IP \\[bu]" for bullet lists in man(7)
pages that Alejandro Colomar recommends in the "Lists" subsection of
https://man7.org/linux/man-pages/man7/man-pages.7.html#STYLE_GUIDE .

For example, this will improve HTML formatting of the first list in
the subsection "Feature test macros understood by glibc" on the page
https://manpages.debian.org/bookworm/manpages/ftm.7.en.html .

Issue reported by Alejandro Colomar <alx at kernel dot org>.