Trailing withspace handling in mhl

If a component has trailing whitespace, e.g., body:component="> ", mhl
now trims that whitespace off when filtering blank text lines. Also add
a rtrim flag to mhl. This flag removes trailing whitespace in filtert text.

The implementation was done by David Levine <levinedl@acm.org> for nmh
commit ea0a6d8112a918809bd03d8126411040d22f2bb8
commit 1aa8f3e11e6d83ee4806abaa132ab9466f02ca5f