drop -Dbpfs=userhacks and make recompilation of bpf files consistent
Turns out that only the initial ninja run compiles everything. When we
do a change in a .bpf.c file not in -Dbpfs, meson doesn't detect it
and thus doesn't recompile it.
Ensure the -Dbpfs flag (and filter) matters only for the installation.
This allows users who wants to create a userhack to create it, and
recompile it without requesting the full -Dbpfs=userhacks to be set,
with the risk of installing unwanted files.
And so we can now remove -Dbpfs=userhacks entirely.
Fixes #14 (closed)