Fix spurious creation of '=' file (#98)

The following is quoted from Marcin Cieślak [*]:
When running under FreeBSD /bin/sh (and not ksh) we get spurious
file named '=' created in the root. This is because the "checksh"
function runs /bin/sh -c '(( .sh.version >= 20111111 ))' which
produces a "=" file with /bin/sh as a side effect.

Fixes https://github.com/ksh93/ksh/issues/13

bin/package,
src/cmd/INIT/package.sh:
- Fix the creation of a spurious '=' file by making sure the shell
  has support for (( ... )) expressions.

.gitignore:
- Remove the '=' file entry since it no longer has a purpose.

[*]: https://bsd.network/@saper/103196289917156347

.gitignore

@@ -1,5 +1,4 @@
 # Project-specific files
-=
 arch
 tgz
 lcl