Introduce APIs for splitting/joining strings

Introduce APIs for splitting/joining strings

This introduces a few new APIs for dealing with strings.
One to split a char * into a char **, another to join a
char ** into a char *, and finally one to free a char **

There is a simple test suite to validate the edge cases
too. No more need to use the horrible strtok_r() API,
or hand-written code for splitting strings.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>