Fix regression: Allow FIFO / process substitution for --read-batch

[seks99x]

V3.5.0 in batch.c commit 1604890 introduced a strict S_ISREG check for --read-batch argument paths. This inadvertently breaks bash process substitution (e.g., <(...)), which passes file descriptors as FIFOs (S_IFIFO).

Testing against the 3.4 branch succeeds, but fails on the current 3.5.0dev branch:

Bash

# rsync 3.4.x (Success)
./rsync --read-batch=<(cat reg) /tmp/rsync_test_dir/src/test
skipping non-regular file "test"

# rsync 3.5.0dev (Regression)
./rsync --read-batch=<(cat reg) /tmp/rsync_test_dir/src/test 
Batch file "/dev/fd/63" is not a regular file
rsync error: error in file IO (code 11) at batch.c(281) [Receiver=3.5.0dev-ge8c79d2d]

The Fix:
Modified the check in batch.c open_batch_file() to permit S_ISFIFO alongside S_ISREG.

[seks99x]

the PR wont pass the CI without merging PR #1054