check-help-strings.pl: minor updates

Include the full path to the helpfile in the output message, when
relevant.
helpfiles: remove empty helpfiles
helpfiles: fix filenames referenced by calls to show_help()
btl vader: removed unused helpfile

vader does not invoke show_help() at all -- all the topics in this
helpfile were copied from the original sm btl helpfile.
btl_openib_component.c: remove inline logic and use 2 calls to show_help

The contrib/check-help-strings.pl gets confused if the topic is an
inline logic check, so separate it into two calls to show_help.
btl_sm_component.c: update outdated topic for call to show_help
README: Document the --with-usnic configure switch

cmr=v1.8.2:reviewer=rhc