Fix bugs related to 'uname -d' in the 'uname' builtin (#251)

This commit fixes a bug in the ksh uname builtin's -d option that could
change the output of -o (I was only able to reproduce this on Linux):
    $ builtin uname
    $ uname -o
    GNU/Linux
    $ uname -d
    (none)
    $ uname -o
    (none)
I identified this patch from ksh2020 as a fix for this bug:
<https://github.com/att/ast/pull/1187>
The linked patch was meant to fix a crash in 'uname -d', although I've
had no luck reproducing it: <https://github.com/att/ast/issues/1184>

src/lib/libcmd/uname.c:
- Pass correct buffer to getdomainname() while executing uname -d.

src/cmd/ksh93/tests/builtins.sh:
- Add a regression test for the reported 'uname -d' crash.
- Add a regression test for the output of 'uname -o' after 'uname -d'.
- To handle potential crashes when running the regression tests in older
  versions of ksh, fork the command substitutions that run 'uname -d'.

src/lib/libcmd/uname.c

@@ -493,11 +493,14 @@ b_uname(int argc, char** argv, Shbltin_t* context)
 		if (flags & OPT_domain)
 		{
 			if (!*(s = astconf("SRPC_DOMAIN", NiL, NiL)))
+			{
 #if _lib_getdomainname
-				getdomainname(s, sizeof(buf));
+				getdomainname(buf, sizeof(buf));
+				s = buf;
 #else
 				/*NOP*/;
 #endif
+			}
 			output(OPT_domain, s, "domain");
 		}
 #if _mem_m_type_utsname