libthr: fix warnings at WARNS=6

Fix warnings about the following when WARNS=6 (which I will commit soon):

• casting away const
• no previous 'extern' declaration for non-static variable
• others as explained by #pragmas and comments
• unused parameters

The last is the only functional change.

Reviewed by: kib
MFC after: 3 days
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D10808