debian/rules: Enable parallel building.

See `man dh` and `man dpkg-buildflags` for more information. The user
has to enable for example in `/etc/dpkg/buildflags.conf` how many jobs
can be build in parallel.
Updating too short description for pcmanfm-dbg.

Lintian says the description should not be one-line long.
Updating changelog file to actual upstream version (NMU).