m_config: allow not allocating option struct, and use it

In cases we're just listing options or checking their values (as it
happens with -vo/-vf etc. suboption parsing), we don't need to allocate
abd initialize the actual option struct. All we're interested in is
the list of options.
m_config: remove unused fields

We don't need that anymore, because the old way to get default values
by recreating the m_config is gone.
m_config: slightly improve name handling
mplayer: make --length relative to actual start of file, instead of 0

We always use the file's timestamps, so the start time can be easily
something different from 0. Make the --length option respect this.