usnic provider: address issues from PR 197

Signed-off-by: Reese Faucette <rfaucett@cisco.com>
Merge pull request #202 from rfaucett/usdf2

usnic provider: address issues from PR 197
build: fix build for enable direct

Only one provider can be built in direct mode, so we
keep a count. Also, additional headers need to be
installed.

Include a check to see if the requested provider supports
direct mode, if the user requests to build the provider
as direct.

Quote message in AC_MSG_WARN in square brackets.

Fixes #194

Signed-off-by: Sayantan Sur <sayantan.sur@intel.com>
Merge pull request #200 from sayantansur/fi-direct

build: fix build for enable direct