patch 9.1.1327: filetype: nroff detection can be improved

patch 9.1.1327: filetype: nroff detection can be improved

Problem:  filetype: nroff detection can be improved
Solution: improve nroff detection (Eisuke Kawashima)

- explicitly check roff comments and macros typically found in manpages
- do not try to detect alphabetically-sectioned files, except for n, as
  nroff
    - l: > 'l' happens to be a section for historical reasons
         <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=391977>
    - n: e.g. /usr/share/man/mann/Tcl.n.gz
    - o: unsure (perhaps fedora-specific)
    - p: unsure (perhaps fedora-specific)

closes: #17160

Signed-off-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>