metrics: resolve protocol stat symbols via dlsym

The previous weak-symbol workaround let the module load on musl (which
treats RTLD_LAZY as RTLD_NOW) but introduced a silent failure: weak
undefined references are bound to NULL at our own dlopen time and are
not updated when a later RTLD_GLOBAL dlopen brings the protocol module
in. In any [modules] ordering where metrics comes before pppoe / l2tp /
pptp / sstp / ipoe, the function pointers stay NULL and per-protocol
session metrics silently disappear from both Prometheus and JSON
output, with no log to indicate why.

Replace the weak declarations with a small table and resolve each
protocol's stat_starting/stat_active pair via dlsym(RTLD_DEFAULT, ...)
the first time we render after the module is seen as loaded. dlsym
walks the live global scope at call time, so it picks up symbols
regardless of dlopen order; the resolved pointers are cached so
subsequent scrapes do not re-walk the loader. The five near-identical
render blocks in render_prometheus() and render_json() collapse into
table-driven loops.

libdl is already a transitive dependency of accel-pppd via triton, so
no build-system changes are needed.

Signed-off-by: Denys Fedoryshchenko <denys.f@collabora.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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