split: support optional start value for --numeric-suffixes

split: support optional start value for --numeric-suffixes

Allow changing the --numeric-suffixes start number
from the default of 0.

* src/split.c (next_file_name): Initialize the suffix index
and the output filename according to start value.
(main): Check that the suffix length is large enough for the
numerical suffix start value.
* doc/coreutils.texi (split invocation): Mention it.
* NEWS (New features): Mention it.
* tests/split/numeric: New file. Test --numeric-suffixes[=FROM].
* tests/Makefile.am (TESTS): Reference the new test.