Mencoder should use multithread by default

[EricDHH]

For all ubuntu versions:

What happens: When converting video on ubuntu you see one cpu core under heavy load, but other installed cores in the computer do nothing. Option to multithread in frontends like devede cause no speedup. Lookin over the net you find since 2008 a mencoder-mt version and svn's that use all existing cores for the jobs. Most installed computers have dual or quadcore that can do a conversion job up to 400% faster.

What should happen: Turn multithread on by default, so every multicore machine can benefit from it.