uniq: missing support for obsolete +N syntax in place of -s option

[jfinkels]

The obsolete syntax +N for skipping bytes (superseded by the -s N option) is currently not supported in uutils uniq.

uutils uniq:

$ printf "aaa\nbaa\n" | ./target/debug/uniq +1
./target/debug/uniq: Could not open +1: No such file or directory

GNU uniq:

$ printf "aaa\nbaa\n" | uniq +1
aaa

Notes: Here is the documentation from info uniq:

‘-s N’
‘--skip-chars=N’
     Skip N characters before checking for uniqueness.  Use a null
     string for comparison if a line has fewer than N characters.  If
     you use both the field and character skipping options, fields are
     skipped over first.

     On systems not conforming to POSIX 1003.1-2001, ‘uniq’ supports a
     traditional option syntax ‘+N’.  Although this traditional behavior
     can be controlled with the ‘_POSIX2_VERSION’ environment variable
     (*note Standards conformance::), portable scripts should avoid
     commands whose behavior depends on this variable.  For example, use
     ‘uniq ./+10’ or ‘uniq -s 10’ rather than the ambiguous ‘uniq +10’.

This is causing a test failure in the GNU test file tests/uniq/uniq.pl.