Problem: filetype: some man files are not recognized
(e.g. 1p (POSIX commands))
Solution: update the filetype detection pattern and detect more man
files as nroff (Eisuke Kawashima)
- sections are revised referring to
- debian-12:/etc/manpath.config
- fedora-41:/etc/man_db.conf
- detection logic is improved
- detection test is implemented
closes: vim/vim#17117babdb0554a
Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>