functions/man: fix for embedded fish-* man pages

"man abbr" works in embed-data builds,
but "man fish-faq" doesn't.

This is because it delegates to

	__fish_print_help fish-faq

which skips all lines until the NAME section:

	contains -- $name NAME; and set have_name 1

but the NAME section doesn't exist for this man pages, it only exists
for docs from doc_src/cmds/*.rst.

Let's use the "man" utility instead; this is also what the user
asked for.  Unfortunately we can't use "status get-file | man -l -"
because that's not supported on BSD man.  Note that man displays the
basename of the file, so make sure it looks good.

share/functions/man.fish

@@ -55,10 +55,19 @@ function man --description "Format and display the on-line manual pages"
         end
     end
 
+    set -l tmpdir
     if not set -q argv[2] && status list-files "man/man1/$argv[1].1" &>/dev/null
-        __fish_print_help $argv
-        return
+        set tmpdir (__fish_mktemp_relative -d fish-man)
+        status get-file "man/man1/$argv[1].1" >$tmpdir/$argv.1
+        set argv $tmpdir/$argv.1
     end
 
     command man $argv
+    set -l saved_status $status
+
+    if set -q tmpdir[1]
+        rm -r $tmpdir
+    end
+
+    return $saved_status
 end