NetBSD build fixes

- Shell scripts: == is specific to bash and ksh. Use = instead.
- Shell scripts: use sh instead of bash if bash functionnality is not used
- Shell scripts: ${var/search/replace} is specific to bash
- sed: The -i option is specific to GNU sed.
- Makefiles: $< outside of generic rules only work in GNU make.
- xdrproc_t() is not universally defined as variadic. Do not specify third
  argument if it is not used
- NetBSD FUSE specific: only include <perfuse.h> in FUSE client code,
  it harms in other locations
- configure: Search for gettext() in libintl as NetBSD stores it there
- Like MacOS X, NetBSD has unmount(2) and not umount(2) (un vs u)

Some other build issues previously included in this change were
removed:
- __THROW macro, addressed in http://review.gluster.com/#/c/7757/
- getmntent() compat shared with MacOS X, in http://review.gluster.com/#/c/7722/

This patchset adds warning fixes for mount_glusterfs

BUG: 764655
Change-Id: I2f1faf8ff96362d3e2baf237b943df619011f1f4
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/7783
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>