netstat.c: fix typo in error message
Add netstat -S/--sctp parameter into help and manpages
Fix unsigned sscanf format

After coverity tool scan, some warnings about signed/unsigned conflicts in
sscanf format string were found. Not a serious problem, but better to be
correct.
Documented metric defaults, made markup of keywords/variables more consistent.
fix default for inet4 (0) and pointer to ip(8)