configure.ac: memshr_create: Detect and use from <sys/mman.h>

configure.ac: memshr_create: Detect and use from <sys/mman.h>

Recent glibc versions do have a wrapper for this function.  It is available via
<sys/mman.h>.  So:
  * Add <sys/mman.h> to the list of header checks.
  * Add a conditional include of <sys/mman.h> to the configure test
    for memshr_create, so that it will pass if the function is there.
  * Add the same conditional include to missing.h, so that we
    get it declared when we build.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>