split(1): add '-c' to continue creating files

Currently, split(1) will clobber any existing output files:

$ split file; ls
xaa xab xac xad
$ split second-file; ls
xaa xab xac xad xae xaf

This patch adds a flag "-c" (mnemonic "create, don't overwrite" or
"continue where you left off"):

$ split file; ls
xaa xab xac xad
$ split -c second-file; ls
xaa xab xac xad xae xaf xag xah xai xaj

Reviewed by: christos
Approved by: kevans
Different Revision: https://reviews.freebsd.org/D38553