libc/libc/rpc: refactor some global variables

• Combine dg_fd_locks and dg_cv into one array.
• Similarly for vc_fd_locks and vc_cv
• Turn some macros into inline functions

This is a mostly cosmetic change to make refactoring these strutures in
a future commit easier.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D42597