join: fix -1 X and -2 Y explicit field numbers

[mknos]

• Default field number is 1 for file1 and file2; this is designated by variable $j1 and $j2 being initialised to zero
• This is meant to be equivalent to join -j 1, which sets $j1 and $j2 to the same thing
• This is also supposed to be equivalent to join -1 1 -2 1
• Fix off-by-one error where the field number returned by get_numeric_arg() is not decremented (internally the field values are zero based)
• This issue was flagged by MS-365-Copilot analysis of the program, and patched by me
• Save the regex capture into a named variable to help clarify that it's talking about a file number and not a field number
• Bump version
• Tested against GNU coreutils

Test: before patch

%perl cat TAB1
AA xyz MR_BOB
XX asd MR_FRED
%perl cat TAB2
AA Friendly
BB Extinct
XX Grumpy
%/usr/bin/join --version
join (uutils coreutils) 0.8.0
%/usr/bin/join TAB{1,2}
AA xyz MR_BOB Friendly
XX asd MR_FRED Grumpy
%perl join TAB{1,2}
AA xyz MR_BOB Friendly
XX asd MR_FRED Grumpy
%/usr/bin/join -1 1 -2 1 TAB{1,2}
AA xyz MR_BOB Friendly
XX asd MR_FRED Grumpy
%perl join  -1 1 -2 1 TAB{1,2}
%perl join  -1 0 -2 0 TAB{1,2}
fields start at 1
join (Perl Power Tools) 1.1
usage: join [-a file_number | -v file_number] [-e string] [-j file_number field]
            [-o list] [-t char] [-1 field] [-2 field] file1 file2

[briandfoy]

changes: fix increment bug in setting the two default offsets to the wrong things.