patch 9.1.1304: filetype: some man files are not recognized

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: #17117

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

src/testdir/test_filetype.vim

@@ -2889,6 +2889,20 @@ func Test_map_file()
   filetype off
 endfunc
 
+func Test_nroff_file()
+  filetype on
+
+  call writefile(['.TH vim 1 "YYYY Mth DD"'], 'Xfile.1', 'D')
+  split Xfile.1
+  call assert_equal('nroff', &filetype)
+  bwipe!
+
+  call writefile(['. /etc/profile'], 'Xfile.1', 'D')
+  split Xfile.1
+  call assert_notequal('nroff', &filetype)
+  bwipe!
+endfunc
+
 func Test_org_file()
   filetype on
 

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1304,
 /**/
     1303,
 /**/