init.sh: avoid Solaris 10 /bin/sh portability problem

init.sh: avoid Solaris 10 /bin/sh portability problem

Solaris 10's /bin/sh does not pass '.' arguments 2..N to the
sourced script:
  $ printf 'echo "$@"\n' > f; /bin/sh -c '. ./f bar'
  $ printf 'echo "$@"\n' > f;    bash -c '. ./f bar'
  bar
tests/init.sh relied on that, accepting a --set-path=DIR argument,
and two tests used that idiom.
* tests/init.sh: Update suggested usage comments.
(path_prepend_): New function, to be used in place of
the --src-path=DIR option.
Disallow empty strings and strings containing ":".
(setup_): Move PATH-prepending code into path_prepend_.
* tests/test-pread.sh: Adapt to new usage.
* tests/test-xalloc-die.sh: Likewise.