It's suppsed to remove the file and then copy, which it wasn't doing.
But no wait, it turns out that the described behaviour in the manual doesn't
agree with POSIX. So we change the above fix and the manual to "try copy, and
if fail, try remove, then copy".
Fix bug where "cp -R" didn't work on read-only directories:
It would make the directory, set the mode, and not be able to write files into it.
Don't bother mmap()ing files of zero length. Was a workaround for a bug in Rhapsody
mmap(), which didn't get along with such files, but makes sense anyway.
Fix race condition where "cp -p" would set the mod time of a file before close()ing
the file, which would update the mod time and therefore screw up the "-p" idea,
except, of course, while running in gdb, which sucked.
Add -f option to usage message in binary and man page. Already documented in man page.
