Allow sessions with /bin/sh

The existing code scans through `programs` to find an acceptable
shell to start; `_program` is the shell that is configured for
the session (line 456). The first shell to be found from that
list, is assigned to `exec` and we'll run that shell.

If the shell found wasn't the one configured (e.g. one of the
other ones from the list) then a warning is printed, but we carry on.
**However**, if the shell found is the **last** one in the list
(i.e. `/bin/sh`) then a warning is printed and the shell does
not start.

If the configured shell is `/bin/sh` this obviously breaks down:
it is found (as the first one in the list!) but still equals the
last one; the warning is printed and nothing runs.

It is unclear **why** `/bin/sh` is not allowed as a shell:
it exists, it's an executable, and it's an interactive shell.

Curiously, configuring the shell as `sh` for the session runs
`/bin/sh` in the end, but just tricks the logic here:
 - `checkProgram("sh")` returns `sh` as string,
 - so the comparison against `/bin/sh` fails,
 - and we can run `sh` .. which is `/bin/sh`.

There's no good reason to forbid `/bin/sh`, so change the check to
**only** fail if no shell was found at all (`exec` stayed empty)
or if the found shell behaves weirdly (is not equal to itself).