Use ASCII RS for separating options in DH_INTERNAL_OPTIONS.

Use ASCII RS for separating options in DH_INTERNAL_OPTIONS.

Since now extra options via dh command line arguments are encouraged, dh will
break when a bit more complex option gets added to DH_INTERNAL_OPTIONS and it
gets misparsed by the debhelper command called from the override. E.g.
debian/rules:

| %:
| 	dh --builddirectory="build dir"
|
| override_dh_install:
| 	dh_install

Will fail with something like:

| ....
| make[1]: Entering directory `............'
| dh_install
| cp: cannot stat `debian/tmp/dir': No such file or directory
| dh_install: cp returned exit code 1
| make[1]: *** [override_dh_install] Error 1

So since DH_INTERNAL_OPTIONS is exclusively for internal use, why not to use an
old good ASCII unrepresentable control character as a separator? So I chose
ASCII 1E - RS Record Separator.

Signed-off-by: Modestas Vainius <modestas@vainius.eu>