mg: Fix Coverity Scan warning: Insecure data handling

[Mark Lumsden]

Here is a diff from Joachim Wiberg's version of mg.

"The strlcpy() function is guaranteed to never copy more than 'len - 1'
bytes, so there is no need to check if we copied more.  This is a bogus
warning since the introduction of strlcpy()."

Tested and seems reasonable. ok?

Index: cinfo.c
===================================================================
RCS file: /cvs/src/usr.bin/mg/cinfo.c,v
retrieving revision 1.18
diff -u -p -r1.18 cinfo.c
--- cinfo.c     19 Mar 2015 21:22:15 -0000      1.18
+++ cinfo.c     9 Mar 2021 20:09:54 -0000
@@ -106,7 +106,6 @@ char *
 getkeyname(char *cp, size_t len, int k)
 {
        const char      *np;
-       size_t           copied;

        if (k < 0)
                k = CHARMASK(k);        /* sign extended char */
@@ -151,8 +150,6 @@ getkeyname(char *cp, size_t len, int k)
                *cp = '\0';
                return (cp);
        }
-       copied = strlcpy(cp, np, len);
-       if (copied >= len)
-               copied = len - 1;
-       return (cp + copied);
+
+       return (cp + strlcpy(cp, np, len));
 }