shell: Fix build on Solaris 9

Ensure dash can build in a default Solaris 9 or older environment:

 - Execute scripts with $SHELL rather than /bin/sh, the latter does not
   support e.g. "if ! .." used by mkbuiltins.

 - /bin/awk does not support ?: syntax, use explicit statements instead.

 - /bin/nl requires no spaces between options and arguments.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>