install: implement `--no-target-directory, -T`

[bruecke]

The install command does not support --no-target-directory, -T yet.

[bruecke]

Since I've been working #3814 lately, I'd like to implement this option as well.

I've already started to derive some test cases from the original GNU install, to understand what has to be done:

#export C=coreutils/target/debug/

rm -rf dir* no-dir* file*
mkdir -p dir1 dir3
touch file file2

${C}install -T file dir1            # fails w/ "cannot overwrite directory 'dir1' with non-directory"
${C}install -T dir1 dir3            # fails w/ "omitting directory 'dir1'"
${C}install -TD file dir1/file/     # fails w/ "cannot create regular file 'dir1/file/': Not a directory"
${C}install -T file -t dir1         # fails w/ "cannot combine --target-directory (-t) and --no-target-directory (-T)"
${C}install -T file -t              # fails and prints usage
${C}install -T file file2 dir1      # fails w/ "extra operand 'dir1'", then prints usage
${C}install -T file file2 dir1 dir3 # fails w/ "extra operand 'dir1'", then prints usage
${C}install -T file dir3/file       # pass

[cakebaker]

Fixed in #7867