libc: allow posix_fallocate in capability mode

[emaste]

Summary

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.

[emaste]

Per discussion on the recent capsicum call, posix_fallocate could require CAP_PWRITE (aka CAP_WRITE | CAP_SEEK) instead of just CAP_WRITE.

[emaste]

Require CAP_PWRITE (CAP_WRITE | CAP_SEEK) for posix_fallocate.