commit 069fdc8 changed most uses of
the syscall package to switch utsname from unsigned to signed (see
069fdc8). Those don't seem to be
impacting the code used here, so either stdlib or golang.org/x/sys/unix
should work for this case.

I chose stdlib's syscall package for this case, in case we'd decide to
move this package to a separate module (and want to limit its dependencies).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

This combines the implementations of the Self function, to allow having
a single GoDoc to document the behavior. The naiveSelf function is kept,
because it's used in unit-tests.

There is a minor change in behavior, as this patch removes the stub for
unsupported platforms (non-linux, windows, freebsd or darwin), which will
now use `os.Args[0]`. The stub was added in 21537b8
to fix compilation of https://github.com/ethereum/go-ethereum on OpenBSD,
which had docker/docker as dependency. It looks like that repository no
longer has this dependency, and as this was only to make the code
compilable, is unlikely to be a problem.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

The Windows, Darwin, and FreeBSD implementations were identical, other
than their GoDoc to be different. Unify them so that we don't have to
maintain separate GoDoc for each.

It's worth noting that FreeBSD also supports Pdeathsig, so could be
using the same implementation as Linux. However, we don't test/maintain
the FreeBSD implementation, and it would require updating to GoDoc to
be more specific about the use of `/proc/self/exe`, so keeping the
status quo for now.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

This package is used in BuildKit, and could be a potential candidate
for moving to a separate module. While it's not too problematic to have
this dependency, the tests only used basic assertions from gotest.tools,
which could be easily re-implemented without the dependency.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Touch-up some GoDoc in the package, and remove "import" comments.

This package is used in BuildKit, and could be a potential candidate
for moving to a separate module. The "import" comments are ignored when
used in go module mode so have little benefit. Let's remove them.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>