update some man pages

*sum tools: -c was added
tr: -d flag was added.
xargs: -n flag was added.
cmp: minor typo (change to lowercase).

Signed-off-by: Hiltjo Posthuma <hiltjo@codemadness.org>

xargs.1

@@ -3,6 +3,8 @@
 xargs \- constuct argument list(s) and execute command
 .SH SYNOPSIS
 .B xargs
+.RB [\-n
+.IR maxargs ]
 .RB [ \-r ]
 .RB [ \-E
 .IR eofstr ]
@@ -25,6 +27,9 @@ newlines, up to the matching double quote.  Any single character, including
 newlines, may be escaped by a backslash.
 .SH OPTIONS
 .TP
+.B \-n maxargs
+Use at most maxargs arguments per command line.
+.TP
 .BI \-r
 Do not run the command if there are no arguments.  Normally the command is
 executed at least once even if there are no arguments.