pthread_cancel.3, string.3, cpuset.7, tcp.7: ffix (mark up page xfrefs using .BR, not .IR)

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

man3/pthread_cancel.3

@@ -73,7 +73,7 @@ Deferred cancelability means that cancellation will be delayed until
 the thread next calls a function that is a
 .IR "cancellation point" .
 A list of functions that are or may be cancellation points is provided in
-.IR pthreads (7).
+.BR pthreads (7).
 
 When a cancellation requested is acted on, the following steps occur for
 .IR thread

man3/string.3

@@ -120,7 +120,7 @@ which does not contain any of bytes in the string
 Return a duplicate of the string
 .I s
 in memory allocated using
-.IR malloc (3).
+.BR malloc (3).
 .TP
 .BI "char *strfry(char *" string );
 Randomly swap the characters in

man7/cpuset.7

@@ -785,7 +785,7 @@ balancing works to ensure that as many CPUs as possible are usefully
 employed running tasks.
 The kernel also performs periodic load
 balancing off the software clock described in
-.IR time (7).
+.BR time (7).
 The setting of
 .I sched_relax_domain_level
 applies only to immediate load balancing.

man7/tcp.7

@@ -1087,7 +1087,7 @@ It can be changed via
 .IR /proc/sys/net/ipv4/tcp_stdurg .
 
 It is possible to peek at out-of-band data using the
-.IR recv (2)
+.BR recv (2)
 .B MSG_PEEK
 flag.