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.1

@@ -5555,7 +5555,7 @@ command and the original positional parameters are restored upon completion.
 Otherwise the positional parameters are unchanged.
 The exit status is the exit status of the last command executed.
 .TP
-\(dg\(dg \f3alias\fP \*(OK \f3\-ptx\fP \*(CK  \*(OK \f2name\fP\*(OK \f3=\fP\f2value\^\fP  \*(CK \*(CK .\|.\|.
+\f3alias\fP \*(OK \f3\-ptx\fP \*(CK  \*(OK \f2name\fP\*(OK \f3=\fP\f2value\^\fP  \*(CK \*(CK .\|.\|.
 .B alias\^
 with no arguments prints the list of aliases
 in the form
@@ -5580,16 +5580,13 @@ A trailing space in
 .I value\^
 causes the next word to be checked for
 alias substitution.
-The obsolete
+With the
 .B \-t
-option is used to set and list tracked aliases.
-The value of a tracked alias is the full pathname
-corresponding to the given
-.IR name .
-The value becomes undefined when the value of
-.SM
-.B PATH
-is reset but the alias remains tracked.
+option, each name is looked up as a command in
+.B $PATH
+and its path is added to the hash table as a 'tracked alias'.
+If no name is given, this prints the hash table. See
+.BR hash.
 Without the
 .B \-t
 option,
@@ -6125,19 +6122,20 @@ The option
 can only be specified as the first option.
 .TP
 \f3hash\fP \*(OK \f3\-r\fP \f2\^\fP\*(CK \*(OK \f3utility\^\fP \*(CK
-The
-.BR hash
-command can be used to display or modify the hash table.
-The hash table is a list of the most recently used programs
-and their locations.
-If
+.B hash
+displays or modifies the hash table with the locations of recently used
+programs. If given no arguments, it lists all command/path associations
+(a.k.a. 'tracked aliases') in the hash table. Otherwise,
+.B hash
+performs a
+.B PATH
+search for each
 .I utility\^
-is supplied,
-.BR hash
-will add that utility to the hash table.
-If
+supplied and adds the result to the hash table.
+The
 .B \-r
-is selected, the hash table is reset.
+option empties the hash table. This can also be achieved by resetting
+.BR PATH.
 .TP
 .PD 0
 \f3hist\fP \*(OK \f3\-e\fP \f2ename\^\fP \ \*(CK \*(OK \f3\-nlr\^\fP \*(CK \*(OK \f2first\^\fP \*(OK \f2last\^\fP \*(CK \*(CK