install: fix issue #3814

[bruecke]

This pull request fixes the the issue mentioned in #3814, so installing multiple files using the "create leading dirs" option combined with the target parameter is now working and the GNU test is passing.

Furthermore this PR fixes more issues:

1. The verbose option, when using -D, was printing all directories to stdout, instead of only the missing ones.
2. Also installing only a single file using -Dt is now working as expected.
3. Installing a single file into a destination file, but the source being a directory, did not fail with the correct error message.

• [ ] Copy GNU test case
• [ ] Add testcase to verify the correct behaviour