libproc: Add support for some proc_attach() flags.

libproc: Add support for some proc_attach() flags.

This change adds some handling for the equivalent of Solaris' PGRAB_*
flags. In particular, support for PGRAB_RDONLY is needed to avoid a
nasty deadlock: dtrace(1) may otherwise stop the master process for its
pseudo-terminal and end up blocking while writing to standard output.