maint: avoid strncat warning on GCC

GCC 10.1.1 without optimization gives:

  error: ‘strncat’ argument 2 declared attribute ‘nonstring’
    [-Werror=stringop-overflow=]
    strncat (comment, UT_ID (utmp_ent), utmpsize);

Note the strncat man page says that:
  "src does not need to be null-terminated
   if it contains n or more bytes."
And the POSIX spec says that the second (source) parameter
is an array not a string.
So I think it's incorrect for strncat to require src be a string type.
This constraint seems to be being added to the gcc builtin strncat,
as specifiying -fno-builtin also avoids the warning.
Note specifying any optimization level also avoids the warning.

* src/who.c (make_id_equals_comment): Avoid the issue by using
stpcpy + stzncpy, instead of strcpy + strncat.
This pattern is used elsewhere in who.c

src/who.c

@@ -450,8 +450,8 @@ make_id_equals_comment (STRUCT_UTMP const *utmp_ent)
   size_t utmpsize = sizeof UT_ID (utmp_ent);
   char *comment = xmalloc (strlen (_("id=")) + utmpsize + 1);
 
-  strcpy (comment, _("id="));
-  strncat (comment, UT_ID (utmp_ent), utmpsize);
+  char *p = stpcpy (comment, _("id="));
+  stzncpy (p, UT_ID (utmp_ent), utmpsize);
   return comment;
 }