TOOLS/umpv: create FIFO in user directory

Makes these security measures unnecessary.
input: resolve ~ and similar for --input-file

Because why not.
command: finish hook execution if client fails

Translation: if the (to be added) youtube-dl Lua script crashes, don't
wait forever when opening something.
lua: lua_tostring() on an error value can return NULL

Lua is so clever it allows values that can't be converted to strings, in
which case lua_tostring() returns NULL. Trigger undefined behavior.