rtld-elf/paths.h: Make it usable outside rtld

but still for tightly coupled things like ldd(1)

Rename paths.h to rtld_paths.h.
Add guard for rtld-specific externs declarations.
Add _COMPAT32_BASENAME_RTLD and _COMPAT32_PATH_RTLD.

Reviewed by: arichardson, jhb
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32464