Refactor new b_hash(); better hash table clear (re: d8428a83)

The b_hash() function duplicated much of its code from b_alias(),
while b_alias() retained some code to support being called as
'hash'. There is no reason why 'hash' and 'alias' can't be handled
with a single function, as is the case several other builtins. Note
that option parsing can easily be made dependent on the name the
command was invoked with (in this case, argv[0]=='h').

The new hash builtin's -r option cleared the hash table by
assigning to PATH its existing value, triggering its associated
discipline function (put_restricted() in init.c) which then
actually cleared the hash table. That's a bit of a hack. It's nicer
if we can just do that directly. This requires taking a static
handler function rehash() from init.c, which invalidates one hash
table entry, and making it available to the builtin.

src/cmd/ksh93/bltins/typeset.c,
src/cmd/ksh93/include/builtins.h,
src/cmd/ksh93/include/nval.h,
src/cmd/ksh93/sh/init.c,
src/cmd/ksh93/sh/name.c:
- Merge b_hash() into b_alias().
- The -x option was still uselessly setting the NV_EXPORT flag.
  Exported aliases were in ksh88 but were removed in ksh93.
- Rename rehash() handler function from init.c to nv_rehash
  (avoiding a possible conflict with another rehash() in cd_pwd.c)
  and move it to name.c just above nv_scan(), which it's meant to
  be used with. Make it an extern so typeset.c can use it.
- b_alias(): Replace the PATH assignment by an nv_scan() call to
  clear the hash table directly using the nv_rehash() handler.

src/cmd/ksh93/data/builtins.c:
- POSIX compliance fix: Remove BLT_SPC (special builtin) flag from
  "alias" definition. 'alias' is specified as a regular builtin.
- sh_optalias[]: Fix uninformative -t option documentation.
- sh_opthash[]: Edit for conciseness and clarity.

src/cmd/ksh93/sh.1:
- Edit the 'alias -t' and 'hash' documentation.
- Remove the -- prefix from the 'alias' entry, which indicated that
  it was supposed to be a declaration builtin like 'typeset', with
  assignment-arguments expanding tildes and not being subject to
  field splitting. However, my testing shows that 'alias' has never
  actually behaved that way on ksh93. Even adding the BLT_DCL flag
  in data/builtins.c doesn't seem to change that.

(cherry picked from commit afa68dca5c786daa13213973e8b0f9bf3a1dadf6)

src/cmd/ksh93/include/builtins.h

@@ -85,7 +85,6 @@ extern int b_builtin(int, char*[],Shbltin_t*);
 extern int b_cd(int, char*[],Shbltin_t*);
 extern int b_command(int, char*[],Shbltin_t*);
 extern int b_getopts(int, char*[],Shbltin_t*);
-extern int b_hash(int, char*[],Shbltin_t*);
 extern int b_hist(int, char*[],Shbltin_t*);
 extern int b_let(int, char*[],Shbltin_t*);
 extern int b_read(int, char*[],Shbltin_t*);

src/cmd/ksh93/include/nval.h

@@ -281,6 +281,7 @@ extern Namval_t		*nv_open(const char*,Dt_t*,int);
 extern void 		nv_putval(Namval_t*,const char*,int);
 extern void 		nv_putv(Namval_t*,const char*,int,Namfun_t*);
 extern int		nv_rename(Namval_t*,int);
+extern void		nv_rehash(Namval_t*,void*);
 extern int		nv_scan(Dt_t*,void(*)(Namval_t*,void*),void*,int,int);
 extern char 		*nv_setdisc(Namval_t*,const char*,Namval_t*,Namfun_t*);
 extern void		nv_setref(Namval_t*, Dt_t*,int);

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-06-08"
+#define SH_RELEASE	"93u+m 2020-06-10"