configure.ac: prepare for 0.18.1
decoder/ffmpeg: support opus
Construct a Null AllocatedPath if the filename conversion into UTF8 failed
event/ServerSocket: fix get_remote_uid() error value

Must return -1 on error, not 0.  0 is root.
Client: assume uid==0 is local socket

A negative uid value means it's not a "local socket" (PF_LOCAL).
uid==0 means user "root" connected.
release v0.18.18
Merge tag 'v0.18.18' into v0.19.x
release v0.19.4