Various pages: tfix: Use Oxford comma

Found using:
$ pcregrep -rnM "^\.[B|I]R .*,\n\.[B|I].*[^,]\nor" man? \
  |grep ^man \
  |sort;

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

man2/mmap.2

@@ -563,7 +563,7 @@ was 0.
 .I flags
 contained none of
 .BR MAP_PRIVATE ,
-.B MAP_SHARED
+.BR MAP_SHARED ,
 or
 .BR MAP_SHARED_VALIDATE .
 .TP

man3/aio_return.3

@@ -53,7 +53,7 @@ If the asynchronous I/O operation has completed, this function returns
 the value that would have been returned in case of a synchronous
 .BR read (2),
 .BR write (2),
-.BR fsync (2)
+.BR fsync (2),
 or
 .BR fdatasync (2),
 call.

man3/dbopen.3

@@ -522,7 +522,7 @@ routines may fail and set
 for any of the errors specified for the library routines
 .BR read (2),
 .BR write (2),
-.BR free (3)
+.BR free (3),
 or
 .BR malloc (3).
 .PP

man3/drand48.3

@@ -147,7 +147,7 @@ The value returned by any of the functions
 .BR erand48 (),
 .BR lrand48 (),
 .BR nrand48 (),
-.BR mrand48 ()
+.BR mrand48 (),
 or
 .BR jrand48 ()
 is

man3/posix_spawn.3

@@ -513,13 +513,13 @@ and
 .BR posix_spawnp ()
 functions fail only in the case where the underlying
 .BR fork (2),
-.BR vfork (2)
+.BR vfork (2),
 or
 .BR clone (2)
 call fails;  in these cases, these functions return an error number,
 which will be one of the errors described for
 .BR fork (2),
-.BR vfork (2)
+.BR vfork (2),
 or
 .BR clone (2).
 .PP

man3/xdr.3

@@ -99,7 +99,7 @@ Note: encoded characters are not packed, and occupy 4 bytes each.
 For arrays of characters, it is worthwhile to
 consider
 .BR xdr_bytes (),
-.BR xdr_opaque ()
+.BR xdr_opaque (),
 or
 .BR xdr_string ().
 .PP

man4/sk98lin.4

@@ -248,7 +248,7 @@ If AutoNeg_A is set to
 .IR On ,
 all three values of DupCap_A (
 .IR Half ,
-.I Full
+.IR Full ,
 or
 .IR Both )
 might be stated.
@@ -278,7 +278,7 @@ If AutoNeg_B is set to
 .IR On ,
 all three values of DupCap_B (
 .IR Half ,
-.I Full
+.IR Full ,
 or
 .IR Both )
 might be stated.

man7/symlink.7

@@ -548,7 +548,7 @@ the
 command acts a little differently.
 If you do not specify the
 .IR \-F ,
-.IR \-d
+.IR \-d ,
 or
 .I \-l
 options,