tweak manpages formatting

Most manpages markup NULL and PCAP_ macros with .B, fix a few remaining
files for consistency.

pcap_datalink_val_to_name.3pcap

@@ -37,8 +37,12 @@ const char *pcap_datalink_val_to_description(int dlt);
 .SH DESCRIPTION
 .B pcap_datalink_val_to_name()
 translates a link-layer header type value to the corresponding
-link-layer header type name.  NULL is returned on failure.
+link-layer header type name.
+.B NULL
+is returned on failure.
 .PP
 .B pcap_datalink_val_to_description()
 translates a link-layer header type value to a short description of that
-link-layer header type.  NULL is returned on failure.
+link-layer header type.
+.B NULL
+is returned on failure.

pcap_get_tstamp_precision.3pcap.in

@@ -39,7 +39,11 @@ returns the precision of the time stamp returned in packet captures on the pcap
 descriptor.
 .SH RETURN VALUE
 .B pcap_get_tstamp_precision()
-returns PCAP_TSTAMP_PRECISION_MICRO or PCAP_TSTAMP_PRECISION_NANO, which indicates
+returns
+.B PCAP_TSTAMP_PRECISION_MICRO
+or
+.B PCAP_TSTAMP_PRECISION_NANO,
+which indicates
 that pcap captures contains time stamps in microseconds or nanoseconds respectively.
 .SH SEE ALSO
 pcap(3PCAP),

pcap_tstamp_type_val_to_name.3pcap

@@ -36,10 +36,14 @@ const char *pcap_tstamp_type_val_to_description(int tstamp_type);
 .SH DESCRIPTION
 .B pcap_tstamp_type_val_to_name()
 translates a time stamp type value to the corresponding time stamp type
-name.  NULL is returned on failure.
+name.
+.B NULL
+is returned on failure.
 .PP
 .B pcap_tstamp_type_val_to_description()
 translates a time stamp type value to a short description of that time
-stamp type.  NULL is returned on failure.
+stamp type.
+.B NULL
+is returned on failure.
 .SH SEE ALSO
 pcap(3PCAP), pcap_tstamp_type_name_to_val(3PCAP)