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>

src/testdir/test_filetype.vim

@@ -2955,11 +2955,21 @@ endfunc
 func Test_nroff_file()
   filetype on
 
-  call writefile(['.TH vim 1 "YYYY Mth DD"'], 'Xfile.1', 'D')
+  call writefile(['.TH VIM 1 "YYYY Mth DD"'], 'Xfile.1', 'D')
   split Xfile.1
   call assert_equal('nroff', &filetype)
   bwipe!
 
+  call writefile(['.Dd $Mdocdate$', '.Dt "DETECTION TEST" "7"', '.Os'], 'Xfile.7', 'D')
+  split Xfile.7
+  call assert_equal('nroff', &filetype)
+  bwipe!
+
+  call writefile(['''\" t'], 'Xfile.3p', 'D')
+  split Xfile.3p
+  call assert_equal('nroff', &filetype)
+  bwipe!
+
   call writefile(['. /etc/profile'], 'Xfile.1', 'D')
   split Xfile.1
   call assert_notequal('nroff', &filetype)

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1327,
 /**/
     1326,
 /**/