cli: drop --enableplugin.

documented in cli_vs_yum.
cli: re-enable recognized options in --help.
plugins: --disableplugin works again.
plugins: redo the plugin config loading, with doc.
In an afterthought, why encourage clients to behave abnormally.
New version: 0.4.11
build: 0.4.11-1
doc: two new FAQs about MD synchronization.
cli: fix another ridiculous capitalization.
config: make sure faulty values do not propagate into the conf. (RhBug:1048488)
drpm: add config options + docs, load updateinfo.xml
drpm: download delta packages if possible
get_package_target(): support the "donecb" callback
drpm: spawn delta rebuild jobs, report failures
drpm: display delta rebuilds as they finish
drpm: fall back to rpm download on drpm errors
Add "throttle" and "bandwidth" options.