lavf/ftp: make response parsing more RFC compliant

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavf/ftp: explicitly enable UTF-8

Most FTP servers enable UTF-8 by default, but it is not required by the standard.
Enabling it manually makes ffmpeg implementation more consistent when server
doesn't enable it by default.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavf/ftp: always treat all response codes >= 500 as error

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
Merge remote-tracking branch 'lukaszmluki/master'

* lukaszmluki/master:
  lavf/ftp: always treat all response codes >= 500 as error
  lavf/ftp: explicitly enable UTF-8
  lavf/ftp: make response parsing more RFC compliant

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