cache: use sendfile() instead of a pair of read() + write()

sendfile() does the same job and avoids to copy the content into userland
and back. One has to define NO_SENDFILE in case the OS (kernel / libc)
does not supported. It is disabled by default on non-linux environemnts.
According to the glibc, sendfile64() was added in Linux 2.4 (so it has
been there for a while) but after browsing over the mapage of FreeBSD's I
noticed that the prototype is little different.

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

Makefile

@@ -29,6 +29,7 @@ DOC_PDF  = $(patsubst %.txt,%.pdf,$(MAN_TXT))
 # j, z, t. (representing long long int, char, intmax_t, size_t, ptrdiff_t).
 # some C compilers supported these specifiers prior to C99 as an extension.
 #
+# Define HAVE_LINUX_SENDFILE to use sendfile()
 
 #-include config.mak