tail: obsolete syntax rejects -cf and accepts more than one f #15517

tail's obsolete f suffix is handled differently from GNU in two ways:

1. tail -cf FILE (last 10 bytes, then follow) is rejected. clap reads it as -c f, the count f fails to parse, and that error is returned before the obsolete-syntax parser is tried.
2. More than one f is accepted (-5ff, -cff, +cff), while GNU accepts only one.

$ printf 'abcdefghijklmnop\n' > t.txt

$ tail -cf t.txt                  # GNU coreutils 9.12
hijklmnop
(keeps following)
$ ./target/debug/tail -cf t.txt   # uutils main (bf9513b16)
tail: invalid number of bytes: 'f'
$ echo $?
1

$ tail -5ff t.txt                 # GNU coreutils 9.12
tail: option used in invalid context -- 5
$ echo $?
1
$ ./target/debug/tail -5ff t.txt  # uutils main
abcdefghijklmnop
(keeps following)

The same happens with printf 'abcdefghijklmnop\n' | tail -cf on stdin. tail -c f (separate argument) should still be an invalid count, as in GNU.

Found with behave, which recorded GNU tail's behaviour and replayed it against uutils.