kdump: decode SHM_ANON as first arg to legacy shm_open(2)

The first argument to shm_open(2) as well as shm_open2(2) may be a path or
SHM_ANON. Decode SHM_ANON, at least- paths will show up as namei results in
kdump output, which may be sufficient; in those cases, we'll have printed an
address.

Future commits will add support for shm_open2() to libsysdecode/truss/kdump.

Reported by: kaktus
MFC after: 3 days