Update shell.c and variables.c: add --import-function flag

* See https://svnweb.freebsd.org/ports/head/shells/bash/files/extrapatch-import-functions?view=markup&pathrev=369341
* Compile option `IMPORT_FUNCTIONS_DEF` is set to 0 (false) as default
* Add import-functions.patch
* Remove build/ostype.h for clean build
Add README.md