Log: various: improve fencer connection messages

Previously, log messages around fencer connections were inconsistent.

This attempts to make them more consistent by: having stonith_api_signon() log
only at debug level, letting the callers log at a level appropriate to the
situation using the return code; functionizing retrying a connection; and
using similar wording across clients.

This also does a bit of refactoring for better error checking and improved
efficiency.