sh_type(): Do not set POSIX mode when invoked as su

On Linux, the 'su' program sets $0 to '-su' when doing 'su -' or
'su - username'. When ksh is the target account's default shell,
this caused ksh to consider itself to be launched as a standard
POSIX sh, which (among other things) disables the default aliases
on interactive shells. This caused confusion for at least one user
as they lost their 'history' alias after 'su -':

https://www.linuxquestions.org/questions/slackware-14/in-current-with-downgrade-to-ksh93-lost-the-alias-history-4175703408/

bash does not consider itself to be sh when invoked as su, so ksh
probably shouldn't, either. The behaviour was also undocumented,
making it even more surprising.

src/cmd/ksh93/sh/init.c: sh_type():
- Only set the SH_TYPE_POSIX bit if we're invoked as 'sh' (or, on
  windows, as 'sh.exe').

Author: McDutchie

src/cmd/ksh93/sh/init.c

@@ -1164,16 +1164,16 @@ int sh_type(register const char *path)
 		}
 		break;
 	}
+#if _WINIX
+	if (!(t & SH_TYPE_KSH) && *s == 's' && *(s+1) == 'h' && (!*(s+2) || *(s+2) == '.'))
+#else
+	if (!(t & SH_TYPE_KSH) && *s == 's' && *(s+1) == 'h' && !*(s+2))
+#endif
+		t |= SH_TYPE_POSIX;
 	if (*s++ == 's' && (*s == 'h' || *s == 'u'))
 	{
 		s++;
 		t |= SH_TYPE_SH;
-#if _WINIX
-		if (!(t & SH_TYPE_KSH) && (!*s || *s == '.'))
-#else
-		if (!(t & SH_TYPE_KSH) && !*s)
-#endif
-			t |= SH_TYPE_POSIX;
 		if ((t & SH_TYPE_KSH) && *s == '9' && *(s+1) == '3')
 			s += 2;
 #if _WINIX