umtx: Expose struct abs_timeout to the rest of the kernel.

Add umtx_ prefix to all abs_timeout facility and add declaration for it.
For consistency with others abs_timeout mark inline abs_timeout_init2.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D31249
MFC after: 2 weeks

(cherry picked from commit 307a3dd35cd48ae39849192e0d7a7d34ac7e6ac8)