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)

NEWS

@@ -4,6 +4,12 @@ For full details, see the git log at:
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-06-11:
+
+- POSIX compliance fix: It is now possible to set shell functions named
+  'alias' or 'unalias', overriding the commands by the same names. In
+  technical terms, they are now regular builtins, not special builtins.
+
 2020-06-10:
 
 - The 'hash' utility is now a regular builtin instead of an alias to