build: add die.h; avoid new warnings from GCC 7

build: add die.h; avoid new warnings from GCC 7

* src/die.h (die): New file/function from grep.
Note: we expect this file to migrate to gnulib.
* src/csplit.c: Include die.h.
(check_format_conv_type): Use die in place of error-nonzero;break;
* src/install.c (strip): Likewise.
* src/nl.c (proc_text): Likewise.  This also suppresses a new warning
from GCC 7's -Werror=strict-overflow.
* src/tail.c (parse_options): Likewise.
* src/basename.c (main): Adjust "fall through" comment
so that GCC 7's -Wimplicit-fallthrough honors it.
* src/cp.c (main): Add a "fall through" comment.
* src/ls.c (gobble_file): Likewise.
(get_funky_string): Adjust a "fall through" comment so it is
recognized.
* cfg.mk (exclude_file_name_regexp--sc_system_h_headers): Add die.h
to this list of exempt src/*.h files.