Answer

Informix server gives error -951 while creating sysmaster database and following message is seen in the online.log

14:17:40 On-Line Mode
14:17:40 Building 'sysmaster' database ...
14:17:40 Get Shadow Password for user [informix] failed!
14:17:40 Check for password aging/account lock-out.
14:17:40 listener-thread: err = -951: oserr = 0: errstr = informix@nke-lnx-sci-p001: Incorrect password or user informix@nke-lnx-sci-p001 is not known on the database server.

Check /etc/fstab file to find the options which are used for mounting the device where Informix is installed. If nosuid option is used please remount the device without nosuid option.

Below is the sample entry from /etc/fstab. Informix was installed under /manh directory.

cat /etc/fstab | grep manh
/dev/vg02/lvmanh /manh ext3 nosuid 1 2

nosuid prevents set-user-identifier (suid) or set-group-identifier bits to take effect.

$INFORMIXDIR/bin/oninit binary has s-bit set which allows users to run an executable with the permissions of the executable's owner or group.

-rwsr-sr-- 1 root informix 22116667 Jan 9 13:54 oninit

If the file system where INFORMIX is installed is mounted with -nosuid option it prevents oninit to be run with effective userid of root.
ps -ef | grep oninit lists 'oninit' processes as owned by informix and not root. So this was preventing sysmaster to be created.

Unmount and mount the file system where INFORMIX is installed with default settings.