head: rework handling of non-seekable files #6869

Job	Run time
Build the fuzzers	2m 3s
Run the fuzzers (fuzz_test, true)	2m 35s
Run the fuzzers (fuzz_echo, true)	2m 21s
Run the fuzzers (fuzz_expr, true)	2m 28s
Run the fuzzers (fuzz_printf, false)	2m 24s
Run the fuzzers (fuzz_date, false)	1m 39s
Run the fuzzers (fuzz_seq, false)	1m 33s
Run the fuzzers (fuzz_env, false)	1m 20s
Run the fuzzers (fuzz_split, false)	1m 27s
Run the fuzzers (fuzz_parse_glob, true)	2m 20s
Run the fuzzers (fuzz_cut, false)	1m 22s
Run the fuzzers (fuzz_sort, false)	1m 47s
Run the fuzzers (fuzz_parse_time, true)	2m 14s
Run the fuzzers (fuzz_tr, false)	1m 21s
Run the fuzzers (fuzz_parse_size, true)	1m 15s
Run the fuzzers (fuzz_cksum, false)	1m 42s
Run the fuzzers (fuzz_wc, false)	1m 35s
Run the fuzzers (fuzz_seq_parse_number, true)	2m 20s
	33m 46s

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