tail: speed up -c N for huge N

tail: speed up -c N for huge N

When the user specifies -c N where 2**63 <= N, don’t give
up and use the slow method (which will exhaust memory if
the file is large).  Instead, treat it as N = 2**63 - 1,
since that has equivalent effect.
* src/tail.c (tail_bytes): With -c N and large N, adjust
the code so that lseeks can still be used without
affecting correctness.  Formerly the code gave up and
did a sequential pass through the whole input, which
could easily exhaust memory.