sftp: fix seekPos + file resuming when part file is of size 11

Summary:
previously seekPos would loop over offset==EAGAIN. the returned off_t of
seek is not an error, but the offset or -1. this incorrect handling
of the return value resulted in attempting to seek a file of the size 11
to get stuck in an infinite loop as EAGAIN==11 and so the loop would
always be true. any other file size would have been fine, so the impact
of this is actually super small.

also sync up the expectation and handling a bit more between copy and put
scenarios.
specifically we always seek to the size we (think) the part file has,
instead of letting the libc determine the end. this is in part so
we can simply do an offset==size comparison to check if the seek worked
to the end we expected it to.

the seekPos() helper was removed as it now serves no purpose over calling
lseek directly.

BUG: 417645
FIXED-IN: 20.04

Test Plan:
- create file
- `split -b 11` file to get a segment exactly EAGAIN size
- copy first segment to some other dir as file.part
- open sftp to other dir and copy file there
- copy doesn't get stuck, the file.part is removed, and the resulting file is same as input
- vice versa copy from sftp to local

Reviewers: ngraham, feverfew

Reviewed By: ngraham

Subscribers: bruns, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D27871