Automake treats $(top_srcdir) specially, so a variable that expands to
it won't work.  There were also bugs in the Makefile.am files that broke
`make distcheck`

This avoids having to use a shell for them.

Fixes OpenSMTPD#921

Otherwise, they would fail at delivery time, which would be harder to
debug.

This avoids the quoting problems that arise with %{mda}.

This avoids the risk that a compromised child process could send spoofed
userinfo values.