MFC r324560: allow posix_fallocate in capability mode

posix_fallocate is logically equivalent to writing zero blocks to the
desired file size and there is no reason to prevent calling it in
capability mode. posix_fallocate already checked for the CAP_WRITE
right, so we merely need to list it in capabilities.conf.

Also MFC r324564: allow posix_fallocate in 32-bit compat capability mode

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