copy: fix file concatenation when invoked via COMMAND /C

The COPY command with file concatenation (e.g., `copy /b a+b dest`)
fails with "Source and destination cannot match" when invoked via
`COMMAND /C`, such as when nmake executes makefile commands.

Root cause: The /C handler builds cmd_line by concatenating argv
elements with spaces, always adding a trailing space after the last
argument. In expand_pluses(), `strrchr(cmd_args, ' ')` then finds
this trailing space instead of the space before the destination
filename, causing the destination to be effectively empty.

Example with trailing space from /C:
  Input: "/b msload.com+msbio.cl1 io.sys "
  last_arg = strrchr(..., ' ') -> points to trailing " "
  Output: "/b msload.com  ;msbio.cl1 io.sys "
  Result: first file group has no destination, defaults to source

This bug was introduced in commit 234e221 ("copy: support any amount
of pluses [fixes #94]"). The previous implementation was immune to
trailing spaces because it used forward-searching (strchr from after
the +) rather than backward-searching (strrchr from end of string).

Fix: Strip trailing spaces from cmd_args_bkp before finding last_arg.

Author: tgies

src/command.c

@@ -2162,7 +2162,12 @@ static int expand_pluses(void)
   int len;
   char *p, *p2, *last_arg;
 
+  if (!strchr(cmd_args, '+'))
+    return 0;
   strcpy(cmd_args_bkp, cmd_args);
+  len = strlen(cmd_args_bkp);
+  while (len > 0 && cmd_args_bkp[len-1] == ' ')
+    cmd_args_bkp[--len] = '\0';
   last_arg = strrchr(cmd_args_bkp, ' ');
   if (!last_arg)
     {