Add gen-snmp-subdriver.sh to distribution tarball
Indent scripts/Makefile.am EXTRA_DIST continuation lines

This might be causing a build error on FreeBSD 10.
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)