lib: unstupidify thread debug information

lib: unstupidify thread debug information

the library's thread scheduling functions keep track of the thread
function's name, so far so good.  However, copying the compiler-provided
constant into a buffer inside the thread structure is plain useless.
Also, strip_funcname() was trying to support something that never
happens.

Instead, let's use some bytes here to track where threads are scheduled
from.  Another commit will print that information on crashes.

Ripping out useless stuff:  -64 bytes in the thread structure
Re-add as const ptr:         +8 bytes
Extra debug info:           +12 bytes

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit 3493b7731b750cbc62f00be94b624a08ccccf0b2)