minor cleanup, consistently use local variable where it makes sense,
no real change.
Some session methods checked URIs in the WiredTiger name space in the
session_api code and some in the schema code.  Change to always check
in the session_api code, not the underlying schema function.

Add name-space checks to the session.{compact,truncate} methods, I don't
see any reason for applications to compact or truncate WT objects.

This means we disallow WiredTiger URIs in create, rename, compact, drop
and truncate, but not in open_cursor, salvage, upgrade and verify.

Probably still not quite right, but better.  We should also probably be
checking things like checkpoint targets?
Flag that WT_SESSION.{drop,rename,salvage,truncate,upgrade,verify} all
return EBUSY if there are open cursors on the object.
I messed up the WT_ERR/WT_RET pairs, fix it.
Don't check the truncate URI if it's not set.
Document that direct_io=[data] (O_DIRECT for data files) implies that you
should turn off mmap (mmap=false), and utilities used to copy backup files
should probably also specify O_DIRECT.  Ref #526.