Add ProcessTerminationResult, for better alignment with POSIX

POSIX issue 7, 2018 edition, section 3.303 "Process Termination" states
the following:

> There are two kinds of process termination:
>
> 1. Normal termination occurs by a return from main(), when requested
>    with the exit(), _exit(), or _Exit() functions; or when the last
>    thread in the process terminates by returning from its start
>    function, by calling the pthread_exit() function, or through
>    cancellation.
>
> 2. Abnormal termination occurs when requested by the abort() function
>    or when some signals are received.

Both these cases can be detected by parent processes separately.
Unfortunately, REv2 currently doesn't allow this, as we only provide an
exit code. Implementations such as Buildbarn tend to set the exit code
for abnormal termination to 128+signum, meaning that if a segmentation
fault occurs, an action terminates with exit code 139. This tends to be
fairly confusing for non-expert users.

The goal of this change is to make all of this less confusing. It adds a
new message type named ProcessTerminationResult, which contains the
termination result in a more structured way. It can now either be an
exit code or a signal number. If it turns out that other operating
systems provide even more ways, we can add additional `oneof` cases.

For the exit code in case of normal termination I decided to use an
int64 instead of int32. The reason being that POSIX allows you to get
the full exit code back, using APIs like waitid(). On ILP64 systems, the
exit code may thus be 64 bits.

For the signal in case of abnormal termination I decided to use a
string. The reason being that signal numbers are not covered by POSIX.
Everybody always assumes that 9 is SIGKILL, but POSIX only documents
this as a convention for the command line flags of the `kill` utility.
Not the the actual `SIG*` constants. Adding an enumeration would also be
unwise, as operating systems are free to add their own signals that are
not covered by POSIX (e.g., SIGINFO on BSD).

Fixes: #240