maint: prefer rawmemchr to memchr when easy

maint: prefer rawmemchr to memchr when easy

* bootstrap.conf (gnulib_modules): Add rawmemchr.
* src/csplit.c: Include idx.h.
* src/csplit.c (record_line_starts):
* src/head.c (elide_tail_lines_pipe):
* src/shuf.c (next_line):
* src/split.c (lines_split):
* src/tail.c (pipe_lines):
* src/wc.c (wc_lines):
Prefer rawmemchr to memchr when rawmemchr is easy.
* src/csplit.c (load_buffer):
* src/head.c (struct linebuffer):
Make room for a 1-byte sentinel.