Fix missing config.h in buffer.c

Fix missing config.h in buffer.c

Due to an oversight, splice will never actually be used for i/o.  Someone forgot
to #include "config.h" in lib/buffer.c (in fact almost no files include that
header).  As a result, even though configure detects splice support and puts
HAVE_SPLICE in config.h, buffer.c is always compiled as if there is no splice
support.

Also add #include "config.h" to fuse.c and fuse_lowlevel.c.  These currently
include it indirectly through fuse_misc.h, but we don't want to depend on that.

Reported by Matthew Gabeler-Lee