Revert <> redir FD except in posix mode (re: eeee77ed, 60516872)

eeee77ed implemented a POSIX compliance fix that caused a potential
incompatibility with existing ksh scripts; it made the (rarely
used) read/write redirection operator, <>, default to file
descriptor 0 (standard input) as POSIX specified, instead of 1
(standard output) which is traditional ksh93 behaviour. So ksh
scripts needed to change all <> to 1<> to override the new default.

This commit reverts that change, except in the new posix mode.

src/cmd/ksh93/sh/lex.c:
- Make FD for <> default to 0 in POSIX mode, 1 otherwise.

src/cmd/ksh93/tests/io.sh:
- Revert <> regression test changes from 60516872; we no longer
  need 1<> instead of <> in ksh code.

src/cmd/ksh93/sh/lex.c

@@ -601,7 +601,7 @@ int sh_lex(Lex_t* lp)
 						c  |= SYMPIPE;
 					else if(c=='<' && n=='>')
 					{
-						lp->digits = 0;
+						lp->digits = sh_isoption(SH_POSIX) ? 0 : 1;
 						c = IORDWRSYM;
 						fcgetc(n);
 						if(fcgetc(n)==';')