fix POSIX shared memory support, conditioned by --enable-posix-shm

* Probe for POSIX shared memory after probing for SysV IPC because
  otherwise SysV IPC always takes over.
* Only do this if --enable-posix-shm was passed to configure.
  This is to maintain backwards compatibility with before this change
  when the probe was never effective.
* Fix apr_shm_attach for when POSIX shared memory is used.
* Encode the shm name passed to shm_open and shm_unlink to comply
  with the functions' requirement on the format.  Without this,
  apache httpd fails trying to create shm with slashes in the name.

Note, this commit does not include changes to files regenerated
by buildconf.
[generated files] fix POSIX shared memory support

Files generated by buildconf.  It was necessary to regenerate
configure because configure.in changed.
create POSIX shared memory with mode 0600, instead of 0644

SysV shm segments are created with mode 0600.  Now the /dev/shm files
backing the POSIX shm have the same protection.
Merge pull request #1 from fastly/jozef/posix-shm

fix POSIX shared memory support, conditioned by --enable-posix-shm