We still need to add an additional capture writer so that we can keep the
build-id around, but this gets the mechanics in place to handle the
PERF_EVENT_MMAP2 event type.
This brings together the two libraries back into one now that the whole
design is pretty well sorted out. They depend on roughly the same libraries
anyway and it's way easier of the single library can both read and write
the capture files (along with bringing in libsysprof-capture symbols in
a single place).
