anvil: Use fifo flag to detect shared FIFO connections from master

anvil: Use fifo flag to detect shared FIFO connections from master

The master process passes two pipes to anvil as listen fds: nonblocking_fd
for KILL notifications from the master itself, and blocking_fd shared by
all service processes for CONNECT/DISCONNECT/KILL. Only the first one was
detected as the master FIFO via listen_fd == MASTER_LISTEN_FD_FIRST, so
connections from service processes arriving via blocking_fd were
classified as regular admin connections.

This had no visible effect previously, but later commits restrict
CONNECT/DISCONNECT/KILL to the shared FIFO connection type, which would
reject these commands from service processes.

Use conn->fifo instead, which is TRUE for both master pipes and never
set for unix listeners that anvil currently has.