lavf/libssh: add private_key option

Allows to specify private key to use during authorization.

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavf/libssh: factorize open_file function

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavf/libssh: factorize file_stat function

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavf/libssh: factorize create_ssh_session function

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavf/libssh: factorize create_sftp_session function

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavf/libssh: add av_cold attributes

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavf/libssh: rename context variable from s into libssh

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavf/libssh: fix seek with whence==SEEK_CUR

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavf/ftp: remove double space

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
Merge remote-tracking branch 'lukaszmluki/master'

* lukaszmluki/master:
  lavf/ftp: remove double space
  lavf/libssh: fix seek with whence==SEEK_CUR
  lavf/libssh: rename context variable from s into libssh
  lavf/libssh: add av_cold attributes
  lavf/libssh: factorize create_sftp_session function
  lavf/libssh: factorize create_ssh_session function
  lavf/libssh: factorize file_stat function
  lavf/libssh: factorize open_file function
  lavf/libssh: add private_key option

Merged-by: Michael Niedermayer <michaelni@gmx.at>