lavu/parseutils: allow MM:SS format for duration in av_parse_time()

Fix trac ticket #2258.

Signed-off-by: Senthilnathan M <senthilnathan.maadasamy@gmail.com>
Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
lavu/parseutils: remove spurious "]]" in date format specification
doc/muxers: fix reference to @anchor defined in another manual

In particular, fix warnings:
*** Undefined node `quoting_and_escaping' in @ref (in doc/muxers.texi l. 767)
*** Undefined node `quoting_and_escaping' in @ref (in doc/muxers.texi l. 772)
tests: fix parseutils test after 12a269a5229d3a37be0743fc9655f743ebc44b6e

"12:34" is now valid syntax.
fate/subtitles: add character encoding conversion test.