lib/57865: Curses addch() family does not handle tab correctly.

[achowe%snert.com@localhost]

>Number:         57865
>Category:       lib
>Synopsis:       Curses addch() family does not handle tab correctly.
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Jan 18 12:55:00 +0000 2024
>Originator:     Anthony Howe
>Release:        9.3
>Organization:
>Environment:
NetBSD elf.snert.com 9.3 NetBSD 9.3 (GENERIC) #0: Thu Aug  4 15:30:37 UTC 2022  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64

>Description:
Displaying a string with tabs using addstr() advances the cursor to the next tab stop, a multiple of 8, just fine.

Displaying a string with tabs via repeated calls to addch(), incorrectly handlings tabs.
>How-To-Repeat:
This short program demonstrates the problem:
```
#include <curses.h>
#include <unistd.h>


void
ruler(void)
{
	standout();
	addstr("123456789.123456789.123456789.123456789.\n");
	standend();
}

void
test_addch_tab(const char *s)
{
	for ( ; *s != '\0'; s++) {
		/* Historical Curses handling of TAB would advance to
		 * the next tabstop (a multiple of 8, eg. 0, 8, 16, ...).
		 * See SUS Curses Issue 7 section 3.4.3.
		 */
		addch(*s);
	}
}

int
main(int argc, char **argv)
{
	const char t1[] = "\t9.123\t78\n";
	const char t2[] = "1234\t9.123\t\t567\n";
	const char t3[] = "Append tab string:";

	if (initscr() == NULL) {
		return 1;
	}

	addstr("\nTab behaviour with addstr() normal.\n");
	ruler();
	addstr(t1);
	addstr(t2);
	addstr(t3); addstr(t1);

	addstr("\nTab behaviour with addch() broken.\n");
	ruler();
	test_addch_tab(t1);
	test_addch_tab(t2);
	test_addch_tab(t3); test_addch_tab(t1);

	refresh();
	sleep(10);
	endwin();

	return 0;
}
```
>Fix: