lua: rename some API functions

send_command     -> command
send_commandv    -> commandv
get_timer        -> get_time
property_get     -> get_property
property_get_string -> get_property_osd
getopt           -> get_opt
lua: change error behavior

Return the error Lua-style, instead of raising it as Lua error. This is
better, because raising errors is reserved for more "fatal" conditions.
Pretending they're exceptions and trying to do exception-style error
handling will just lead to pain in this language.
lua: add set_property function
lua: some minor API changes
manpage: document some Lua scripting functions