movenc: Add a new flag for writing global sidx indexes for dash

The double meaning of the faststart flag (moving a moov atom
to the start of files, making them streamable, for non-fragmented
files, vs inserting a global sidx index at the start of files
for fragmented files) is confusing - see 40ed1cbf for
explanation of its origins.

Since the second meaning of the flag hasn't been part of any
libav release yet, just rename it to get rid of the confusion
without any extra deprecation (which wouldn't get rid of the
potential confusion, of users adding -movflags faststart
even for fragmented files, where it isn't needed for making
them "streamable").

This gets back the old behaviour, where -movflags faststart
doesn't have any effect for fragmented files.

Signed-off-by: Martin Storsjö <martin@martin.st>