ubuntu: fix unchecked system() return value

gcc 4.8.2 on ubuntu 14.04 says:

flux.c:401:9: error: ignoring return value of ‘system’, declared with attribute warn_unused_result [-Werror=unused-result]
         (void)system (cmd);
         ^
doc: man page typos, etc.
Merge pull request #26 from garlick/misc-fixes

couple minor tweaks to documentation and one ubuntu compilation issue