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/sh/init.c

@@ -229,15 +229,6 @@ static int		shlvl;
 static int		rand_shift;
 
 
-/*
- * Invalidate all path name bindings
- */
-static void rehash(register Namval_t *np,void *data)
-{
-	NOT_USED(data);
-	nv_onattr(np,NV_NOALIAS);
-}
-
 /*
  * out of memory routine for stak routines
  */
@@ -337,7 +328,8 @@ static void put_restricted(register Namval_t* np,const char *val,int flags,Namfu
 		errormsg(SH_DICT,ERROR_exit(1),e_restricted,nv_name(np));
 	if(np==PATHNOD	|| (path_scoped=(strcmp(name,PATHNOD->nvname)==0)))		
 	{
-		nv_scan(shp->track_tree,rehash,(void*)0,NV_TAGGED,NV_TAGGED);
+		/* Clear the hash table */
+		nv_scan(shp->track_tree,nv_rehash,(void*)0,NV_TAGGED,NV_TAGGED);
 		if(path_scoped && !val)
 			val = PATHNOD->nvalue.cp;
 	}

src/cmd/ksh93/sh/name.c

@@ -2317,6 +2317,15 @@ static int scanfilter(Dt_t *dict, void *arg, void *data)
 	return(0);
 }
 
+/*
+ * Handler function to invalidate a path name binding in the hash table
+ */
+void nv_rehash(register Namval_t *np,void *data)
+{
+	NOT_USED(data);
+	nv_onattr(np,NV_NOALIAS);
+}
+
 /*
  * Walk through the name-value pairs
  * if <mask> is non-zero, then only nodes with (nvflags&mask)==flags