split: port ‘split -n N /dev/null’ better to macOS

* src/split.c (input_file_size): Do not bother with lseek if the
initial read probe reaches EOF, since the file size is known then.
This works better on macOS, which doesn’t allow lseek on /dev/null.
Do not special-case size-zero files, as the issue can occur
with any size file (though /proc files are the most common).
If the current position is past end of file, treat this as
size zero regardless of whether the file has a usable st_size.
Pass through lseek -1 return values rather than using ‘return -1’;
this makes the code a bit easier to analyze (and a bit faster).
Avoid undefined behavior if the size calculation overflows.
(lines_chunk_split): Do not bother with lseek if it would have
no effect if successful.  This works better on macOS, which
doesn’t allow lseek on /dev/null.
* tests/split/l-chunk.sh: Adjust to match fixed behavior.

tests/split/l-chunk.sh

@@ -24,9 +24,10 @@ echo "split: invalid number of chunks: '1o'" > exp
 returns_ 1 split -n l/1o 2>err || fail=1
 compare exp err || fail=1
 
-echo "split: -: cannot determine file size" > exp
-: | returns_ 1 split -n l/1 2>err || fail=1
-compare exp err || fail=1
+rm -f x* || fail=1
+: | split -n l/1 || fail=1
+compare /dev/null xaa || fail=1
+test ! -f xab || fail=1
 
 # N can be greater than the file size
 # in which case no data is extracted, or empty files are written