alter user/role CURRENT_USER

Hello, on the way considering alter role set .., I found that
ALTER ROLE/USER cannot take CURRENT_USER as the role name.

In addition to that, documents of ALTER ROLE / USER are
inconsistent with each other in spite of ALTER USER is said to be
an alias for ALTER ROLE. Plus, ALTER USER cannot take ALL as user
name although ALTER ROLE can.

This patch does following things,
- ALTER USER/ROLE now takes CURRENT_USER as user name.
- Rewrite sysnopsis of the documents for ALTER USER and ALTER  ROLE so as to they have exactly same syntax.
- Modify syntax of ALTER USER so as to be an alias of ALTER ROLE.
  - Added CURRENT_USER/CURRENT_ROLE syntax to both.  - Added ALL syntax as user name to ALTER USER.  - Added IN
DATABASEsyntax to ALTER USER.
 
  x Integrating ALTER ROLE/USER syntax could not be done because of    shift/reduce error of bison.
x This patch contains no additional regressions. Is it needed?

SESSION_USER/USER also can be made usable for this command, but
this patch doesn't so (yet).

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center