Show processes that exec'd but did not fork in `rr ps` (and support them with -p)

When a process exec's but does not fork, it can be confusing for users to know how to attach to the process they want. E.g. consider:

$ cat ls.sh
#!/bin/sh
exec ls
$ chmod +x ./ls.sh
$ rr record ./ls.sh
$ rr ps
PID	PPID	EXIT	CMD
3636445	--	0	./ls.sh
$ rr replay 
(rr) c
Continuing.

Program stopped.
0x0000000070000002 in syscall_traced ()
(rr) bt
#0  0x0000000070000002 in syscall_traced ()
#1  0x00007cebe9815198 in _raw_syscall () at /home/keno/rr/src/preload/raw_syscall.S:120
#2  0x00007cebe980ffb9 in traced_raw_syscall (call=0x681fffa0) at /home/keno/rr/src/preload/syscallbuf.c:379
#3  0x00007cebe9812f38 in sys_fstatat (call=<optimized out>) at /home/keno/rr/src/preload/syscallbuf.c:3716
#4  syscall_hook_internal (call=0x681fffa0) at /home/keno/rr/src/preload/syscallbuf.c:4340
#5  syscall_hook (call=0x681fffa0) at /home/keno/rr/src/preload/syscallbuf.c:4379
#6  syscall_hook (call=0x681fffa0) at /home/keno/rr/src/preload/syscallbuf.c:4363
#7  0x00007cebe980f2e3 in _syscall_hook_trampoline () at /home/keno/rr/src/preload/syscall_hook.S:308
#8  0x00007cebe980f34d in __morestack () at /home/keno/rr/src/preload/syscall_hook.S:443
#9  0x00007cebe980f354 in _syscall_hook_trampoline_48_3d_01_f0_ff_ff () at /home/keno/rr/src/preload/syscall_hook.S:457
#10 0x00007cebe94eef41 in __GI_execve () at ../sysdeps/unix/syscall-template.S:120

i.e. gdb stops in the shell's execve code. Experienced users will know how to find event numbers and get past this, but new users may not. I think we should consider extending rr ps to also show exec events, e.g.

PID	PPID	EXIT	CMD
3636445	--	exec	./ls.sh
3636445.1	--	0	ls

and then let users get to the post-exec state with rr replay -p 3636445.1 etc.

Start with the rr ps and rr replay -p entry points described in the issue, then reproduce the exec-without-fork example. Trace how process rows and replay targets are selected; done means rr ps shows both the pre-exec and post-exec states and rr replay -p 3636445.1 reaches the post-exec state.