override splitting of field names passed to get_field2

There are cases where we want to prevent splitting of field names that
contain dot.  For example, when this is used in
check_hadoop_namenode_state.pl, Cloudera uses the field tag.HAState in
the jmx and we don’t want to split on the dot, otherwise the field
contents never get found.
RHEL/CentOS 6 and higher use em net interface names

update the regex used in isInterface to allow for the name “em”, which
is a valid interface name in RHEL/CentOS6 and higher.
merged patch from Travis Campbell (hcoyote) on GitHub for em interface support from RHEL6 onwards
Merge branch 'hcoyote-master'