Make programPath never return "git-remote-annex" or other known multi-call
program names, which are not git-annex and won't behave like it.
If the git-annex binary gets installed under some entirely other name,
it will still return it.
This change exposed that readProgramFile actually could crash,
which happened before only if getExecutablePath was not absolute
and there was no ~/.config/git-annex/program. So fixed that to catch
exception.
