copy: fix file concatenation when invoked via COMMAND /C

[tgies]

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:

1. Return early if no + in cmd_args (restores old behavior for simple copy commands)
2. Strip trailing spaces from cmd_args_bkp before finding last_arg

The early exit is necessary because stripping trailing spaces from a single-argument copy like copy file (which becomes copy file via COMMAND /C) would leave no spaces, causing a spurious "syntax error".

[stsp]

I believe the CI patch went
in by mistake. Please remove
it and I'll apply.

[tgies]

The CI patch isn't part of this PR -- GitHub is just showing it in the summary because I referenced this PR from a separate commit in my tgies/MS-DOS repo. "tgies added a commit to tgies/MS-DOS that referenced this pull request"

[stsp]

Thanks!