fopen.3: Add subsection headings for each function

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/fopen.3

@@ -147,7 +147,8 @@ at end-of-file, as if preceded the call:
 
     fseek(stream, 0, SEEK_END);
 .fi
-.PP
+.\"
+.SS fdopen()
 The
 .BR fdopen ()
 function associates a stream with the existing file descriptor,
@@ -168,7 +169,8 @@ is closed.
 The result of applying
 .BR fdopen ()
 to a shared memory object is undefined.
-.PP
+.\"
+.SS freopen()
 The
 .BR freopen ()
 function opens the file whose name is the string pointed to by