oscar: Add prefixes to all functions that may conflict with purple
When people build with ./configure --purple=1 --oscar=1, the native
oscar is compiled statically, which should take priority, but sometimes
purple's oscar is used instead and bad things happen, most of it being
due to calling bitlbee's native aim_callhandler instead of the function
with the same name of libpurple's liboscar.so.
