Allow multiple values in *_OLD_CMD
- For many interpreters, there are multiple common shebang patterns (e.g. /usr/bin/perl, /bin/perl, /usr/bin/env perl for perl), each of which require fixing. With this change it's possible to add them all to shebangfix.mk instead of adding *_OLD_CMD to many ports
- Even a single port may use different shebangs for a single interpreter, now it's possible to use shebangfix on them all (possibly with defining custom *_OLD_CMD), while before you had to fix some shebangs with ${REINPLACE_CMD}
While here, add some indentation to shebangfix.mk