Reduce the amount of copy & paste

Instead of having two separate source files for dh-exec-subst and
dh-exec-illiterate, that only differ in the value of
DH_EXEC_CMD_PREFIX, merge them, and have a common dh-exec.simple.c
that can be used to compile both, depending on the value of
DH_EXEC_CMD.

While there, also modified dh-exec-install.c too, to use the same
mechanism, but do a little extra before transferring control.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
t: Make 'distcheck' work

distcheck requires the files under t/pkg-test in the tarball, so
include those. Furthermore, it makes the source directories
unreadable, so copy pkg-test over to a temporary directory, and make
sure it has the right permissions.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>