scanner: better error reporting for CRLF line terminators

scanner: better error reporting for CRLF line terminators

Provide a hint to users that file is coming with CRLF line terminators,
maybe from a non-Linux OS.

Extend scanner.l to provide hint on CRLF in files:

 # file test.nft
 test.nft: ASCII text, with CRLF, LF line terminators
 # nft -f test.nft
 test.nft:1:13-14: Error: syntax error, unexpected CRLF line terminators
 table ip x {
             ^^

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>