Add gen-snmp-subdriver.sh to distribution tarball
cosmetic: Indent scripts/Makefile.am EXTRA_DIST continuation lines
gen-snmp-subdriver.sh: fix option typos

* Use '-M' for MIB directories, to match snmpwalk and the help text.
* Add space before '-c' in snmpwalk (not sure how this worked before)
gen-snmp-subdriver.sh: documentation updates
snmp-ups: add XPPC-MIB for Tripp Lite SU10KRT3/1X