CLEANUP: sym: move symbol resolution out of tools.c

CLEANUP: sym: move symbol resolution out of tools.c

Group symbol resolution and the shared dynamic-loader helpers in sym.c
before extending them with the in-process table: resolve_sym_name(),
the dladdr() and dlsym() wrappers, static-build detection, the executable
path, and the dlopen() interposer which verifies the loaded symbols
through the same helpers. Their declarations, and the backtrace helpers
that report the frames they resolve, move into a new sym.h which their
callers now include.
No functional change.