Remove the options --utf8 and --unicode which had no effect any more
(what they once did should be doable with --subcp). The only use of
corresponding variables left in code was subreader.c code using
sub_utf8 as a flag indicating whether iconv conversion was active.
Change the code to test the existence of iconv context instead.
Conflicts:
DOCS/man/en/options.rst
core/cfg-mplayer.h
sub/sub.c
sub/sub.h
sub/subreader.c
Merged from mplayer2 commit ea7311.
Note: --unicode was already removed