Rename is_XXX_syscall to is_in_XXX_syscall and clarify that the program counter is after the syscall instruction in these cases.
Remove unnecessary templatization
Make (un)traced_syscall_ip always refer to the address of the syscall instruction, to reduce confusion.
Create PaX-compatible post-exec bootstrap sequence

After exec we will perform the following bootstrap sequence:
1) Find an executable syscall instruction in VDSO
2) Use that to perform AutoRemoteSyscalls (instead of patching a syscall
instruction into read-only executable memory)
3) Open file descriptor for I/O to tracee memory (using receive_fd
machinery based on AutoRemoteSyscalls)
4) mmap an "rr page" containing traced and untraced syscalls at
addresses fixed across all tracee processes
5) Use the rr page traced-syscall for future AutoRemoteSyscalls.

Step 4 will later let us using a single SECCOMP filter, set up for the
first tracee, for all tracees.

Step 5 avoids problems if the bootstrap syscall found in step 1 is
later patched by dynamic syscall patching.
Have AutoRemoteSyscalls use AddressSpace::traced_syscall_ip instead of patching in its own syscall instruction.
Print seccomp ptrace events
Move seccomp setup from librrpreload to Task::spawn, using the rr page's untraced syscall instruction for the seccomp filter test.
Fix signedness of original_syscallno.
Use correct Arch-dependent syscall number for restart_syscall
Use correct Arch-dependent syscall numbers in EmuFs
Use correct Arch-dependent size_t for sysctl
Fix alarm test
Add alarm2 test to test SIGALRM signal-handler dispatch during blocking syscall.
Add helper functions to print siginfo
Expect SIGTRAP when single-stepping into a signal handler
Zero-extend 32-bit registers when doing a PTRACE_SETREGS.
Enable basic tests for 32-bit tracees with 64-bit rr.