POSIX compliance fix: make 'unalias' a regular builtin

Both 'alias' and 'unalias' are specified as regular builtins. Among
a few other things, that means it ought to be portable to use these
names for shell functions. But ksh93 disallowed that until now.

src/cmd/ksh93/data/builtins.c:
- Make 'unalias' a regular builtin by removing the BLT_SPC flag.
- (same fix for 'alias' was already done in afa68dca)
- Add the BLT_ENV flag to the 'alias' and 'hash' commands. In
  include/name.h, this flag is commented: "non-stoppable, can
  modify environment". The "non-stoppable" bit seems like a good
  idea: these operations should not be interruptable as that would
  cause an inconsistent state.

src/cmd/ksh93/sh.1:
- Remove the '-', indicating special builtin, from 'alias' entry.
- Minor cosmetic fix: space after the '-' for 'unset'.

(cherry picked from commit a4315d7672204acb543010b4d4916b22dcb9cb08)

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-06-10"
+#define SH_RELEASE	"93u+m 2020-06-11"