OSHMEM: various fixes

- -check-shmem-params is OFF by default. It checks OSHMEM API params and will abort on bad input
- hcoll do not save fallback coll pointers for unsupported collectives.

fixed by Val, Roman, reviewed by Miked/Igor

cmr=v1.7.5:reviewer=ompi-rm1.7
OSHMEM: Fix issue 'OSHMEM passing endpoint==NULL to BTL prepare_src()'

fixed by Igor, reviewed by Miked

fixes #4359