The BSD implementation was under the Advertising Clause,
so we might as well take the opportunity to replace the
implementation entirely with musl's.
Header also rewritten to be a Haiku one; the constants
are left unchanged of course.
Uncomment some functions in unistd.h which glob is using.
git-svn-id: file:///srv/svn/repos/haiku/trunk/current@1419 a95241bf-73f2-0310-859d-f6bbb57e9c96