libthr: fix warnings from GCC when WARNS=6

Fix warnings about:

• redundant declarations
• a local variable shadowing a global function (dlinfo)
• an old-style function definition (with an empty parameter list)
• a variable that is possibly used uninitialized

"make tinderbox" passes this time, except for a few unrelated
kernel failures.

Reviewed by: kib
MFC after: 3 days
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D10870