Ensure returning a EX_* error code in exec_audit.

Fixed a codepath where exec_audit could return EPKG_ERROR, since it
should only use sysexit error codes.
use the __unused macro for argc and argv

In order to avoid scan-build crying a lot about dead increment of argc
and argv after the getopt(3) loop.
Use typedef'ed `pkg_config_t' in for (struct pkg_config).type
Fix an invalid read (discovered by gcc44).
typos fix.
pkg-install exits 0 when requested packages are already installed.