tar: improve quality of diagnostics with incrementals

Inspired by a prototype by Nathan Stratton Treadway in
<http://lists.gnu.org/archive/html/bug-tar/2013-01/msg00000.html>.
* src/incremen.c (read_num): Rewrite by merging read_negative_num
and read_unsigned_num.  Use strtosysint rather than rolling this
stuff ourselves.  Change return type to bool.  All uses changed.
(read_negative_num, read_unsigned_num): Remove.
(read_num, read_timespec, read_incr_db_2): Improve quality of
diagnostics, e.g., by supplying byte offset of error.