linux: Make compat.linux.preserve_vstatus default to 1

From a user point of view, this makes ^T work out of the box.

Reviewed By:	debdrup (man page)
Sponsored By:	EPSRC
Differential Revision:	https://reviews.freebsd.org/D33842

share/man/man4/linux.4

@@ -132,7 +132,7 @@ VSTATUS setting.
 From a user perspective, this makes
 .Va SIGINFO
 work for Linux executables.
-Defaults to 0.
+Defaults to 1.
 .It Va compat.linux.setid_allowed
 Enable handling of set-user-ID and set-group-ID mode bits for the new
 process image file when image is to be executed under Linux ABI.

sys/compat/linux/linux_mib.c

@@ -86,7 +86,7 @@ int linux_ignore_ip_recverr = 1;
 SYSCTL_INT(_compat_linux, OID_AUTO, ignore_ip_recverr, CTLFLAG_RWTUN,
     &linux_ignore_ip_recverr, 0, "Ignore enabling IP_RECVERR");
 
-int linux_preserve_vstatus = 0;
+int linux_preserve_vstatus = 1;
 SYSCTL_INT(_compat_linux, OID_AUTO, preserve_vstatus, CTLFLAG_RWTUN,
     &linux_preserve_vstatus, 0, "Preserve VSTATUS termios(4) flag");