manpage: fixes

Add missing documentation for --quvi-format switch.

Fix description how hw decoding is enabled. The old way currently still
works, but is deprecated.
stream: set default HTTP user agent to "Mozilla/5.0"

Playing vimeo links using quvi support didn't work, even though clive
could. clive is using quvi and curl to download videos from streaming
sites, so if clive works mpv should always work as well. It didn't, and
it turned out that it was due to the user agent. Change the default
from whatever Lavf sends to what clive and cclive use. This will
probably always work, as c(c)live are by the same author as libquvi,
and there's a high chance it has been tested with all the supported
sites.