Tune by using memchr and memrchr.

Tune by using memchr and memrchr.

* .bzrignore: Add string.h.
* admin/merge-gnulib (GNULIB_MODULES): Add memrchr.
* lib/gnulib.mk, m4/gnulib-comp.m4: Regenerate.
* lib/memrchr.c, lib/string.in.h, m4/memrchr.m4, m4/string_h.m4:
New files, from gnulib.
* src/doc.c (Fsnarf_documentation):
* src/fileio.c (Fsubstitute_in_file_name):
* src/search.c (find_newline, scan_newline):
* src/xdisp.c (pos_visible_p, display_count_lines):
Use memchr and memrchr rather than scanning byte-by-byte.
* src/search.c (find_newline): Rename from scan_buffer.
Omit first arg TARGET, as it's always '\n'.  All callers changed.