fcntl.h: warn that bits for O_* flags are scarce

Running out of O_* flag bits will end out ability to make additions that
are source compatible with other operating systems.

Add a warning to coordinate all additions with srcmgr@.

Reviewed by: markj, emaste
Differential Revision: https://reviews.freebsd.org/D50703

(cherry picked from commit 6d0a335aaa81d3ddce95de8d862a23c97bb038a6)