ps(1): Only detect terminal width if stdout is a tty

If stdout isn't a tty, use unlimited width output rather than truncating to
79 characters.  This is helpful for shell scripts or e.g., 'ps | grep foo'.

This hardcoded width has some history: In The Beginning of History[0], the
width of ps was hardcoded as 80 bytes.  In 1985, Bloom@ added detection
using TIOCGWINSZ on stdin.[1]  In 1986, Kirk merged a change to check
stdout's window size instead.  In 1990, the fallback checks to stderr and
stdin's TIOCGWINSZ were added by Marc@, with the commit message "new
version."[2]

OS X Darwin has a very similar modification to ps(1), which simply sets
UNLIMITED for all non-tty outputs.[3]  I've chosen to respect COLUMNS
instead of behaving identically to Darwin here, but I don't feel strongly
about that.  We could match OS X for parity if that is desired.

[0]: https://svnweb.freebsd.org/csrg/bin/ps/ps.c?annotate=1065
[1]: https://svnweb.freebsd.org/csrg/bin/ps/ps.c?r1=18105&r2=18106
[2]:
https://svnweb.freebsd.org/csrg/bin/ps/ps.c?r1=40675&r2=40674&pathrev=40675
[3]:
https://opensource.apple.com/source/adv_cmds/adv_cmds-168/ps/ps.c.auto.html

PR:		217159
Reported by:	Deepak Nagaraj <n.deepak at gmail.com>

bin/ps/ps.c

@@ -194,6 +194,8 @@ main(int argc, char *argv[])
 
 	if ((cols = getenv("COLUMNS")) != NULL && *cols != '\0')
 		termwidth = atoi(cols);
+	else if (!isatty(STDOUT_FILENO))
+		termwidth = UNLIMITED;
 	else if ((ioctl(STDOUT_FILENO, TIOCGWINSZ, (char *)&ws) == -1 &&
 	     ioctl(STDERR_FILENO, TIOCGWINSZ, (char *)&ws) == -1 &&
 	     ioctl(STDIN_FILENO,  TIOCGWINSZ, (char *)&ws) == -1) ||