Re: alter user/role CURRENT_USER

Zero-length identifiers are rejected in scanner so RoleId cannot
be a valid pointer to a zero-length string. (NULL is used as
PUBLIC in auth_ident)

| postgres=# create role "";
| ERROR:  zero-length delimited identifier at or near """"
| postgres=# create role U&"\00";
| ERROR:  invalid Unicode escape value at or near "\00""

As a dirty and quick hack we can use some integer values prfixed
by single zero byte to represent special roles such as
CURRENT_USER.

| RoleId_or_curruser: RoleId                    { $$ = $1; }
|                       | CURRENT_USER          { $$ = "\x00\x01";};
....
| Oid ResolveRoleId(const char *name, bool missing_ok)
| {
|       Oid                     roleid;
|
|       if (name[0] == 0 && name[1] == 1)
|               roleid = GetUserId();

This is ugly but needs no additional struct member or special
logics. (Macros could make them look better.)

|       /* This hack lets us avoid reserving PUBLIC as a keyword*/
|       if (strcmp($1, "public") == 0)

Please let me know the reason to avoid registering new keyword
making the word unusable as an literal identifier, if any?