Several changes to truss.

• Refactor the interface between the ABI-independent code and the ABI-specific backends. The backends now provide smaller hooks to fetch system call arguments and return values. The rest of the system call entry and exit handling that was previously duplicated among all the backends has been moved to one place.
• Merge the loop when waiting for an event with the loop for handling stops. This also means not emulating a procfs-like interface on top of ptrace(). Instead, use a single event loop that fetches process events via waitid(). Among other things this allows us to report the full 32-bit exit value.
• Use PT_FOLLOW_FORK to follow new child processes instead of forking a new truss process for each new child. This allows one truss process to monitor a tree of processes and truss -c should now display one total for the entire tree instead of separate summaries per process.
• Use the recently added fields to ptrace_lwpinfo to determine the current system call number and argument count. The latter is especially useful and fixes a regression since the conversion from procfs. truss now generally prints the correct number of arguments for most system calls rather than printing extra arguments for any call not listed in the table in syscalls.c.
• Actually check the new ABI when processes call exec. The comments claimed that this happened but it was not being done (perhaps this was another regression in the conversion to ptrace()). If the new ABI after exec is not supported, truss detaches from the process. If truss does not support the ABI for a newly executed process the process is killed before it returns from exec.
• Along with the refactor, teach the various ABI-specific backends to fetch both return values, not just the first. Use this to properly report the full 64-bit return value from lseek(). In addition, the handler for "pipe" now pulls the pair of descriptors out of the return values (which is the true kernel system call interface) but displays them as an argument (which matches the interface exported by libc).
• Each ABI handler adds entries to a linker set rather than requiring a statically defined table of handlers in main.c.
• The arm and mips system call fetching code was changed to follow the same pattern as amd64 (and the in-kernel handler) of fetching register arguments first and then reading any remaining arguments from the stack. This should fix indirect system call arguments on at least arm.
• The mipsn32 and n64 ABIs will now look for arguments in A4 through A7.
• Use register %ebp for the 6th system call argument for Linux/i386 ABIs to match the in-kernel argument fetch code.
• For powerpc binaries on a powerpc64 system, fetch the extra arguments on the stack as 32-bit values that are then copied into the 64-bit argument array instead of reading the 32-bit values directly into the 64-bit array.

• Have tested truss -f on a fork test app on amd64.
• Have run truss against a syscall exerciser test program on amd64 and i386.
• Need to get more testing of other platforms before this can be committed.