Fix inconsistent behavior in basic_ostream::seekp () and basic_istream::seekg ()

[Marshall Clow]

Basically, each of these have two methods:

	basic_ostream<charT,traits>& seekp(pos_type pos);
	basic_ostream<charT,traits>& seekp(off_type off, ios_base::seekdir dir);

and one sets the failure bit of the stream on failure, and the other does not.
[ Note that the standard requires us to set the failure bit on the first call, and is silent on the second. I have opened an LWG issue to resolve this. ]
Make the second version set the failbit, and add tests.

Also, the ostream calls were lacking a sentry object.

For basic_istream, there were two calls, as well:

	basic_istream<charT,traits>& seekg(pos_type pos);
	basic_istream<charT,traits>& seekg(off_type off, ios_base::seekdir dir);

and they are both specified to set the failbit on failure, and only the first one did.
Fix that, and add a test to be sure.


-- Marshall

Marshall Clow     Idio Software   <mailto:mclow.lists at gmail.com>

A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait).
        -- Yu Suzuki
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