Add string-utils convenience library

The new src/common/string-utils/libstring-utils.la library has a few
utility functions to manipulate and check strings. See string-utils.h
for more details.

One such function which should prove useful in this codebase is
strutils_split(): it splits an input string with a given delimiter and
returns a NULL-terminated array of newly allocated substrings. This
function supports escaping the delimiter with `\` in the input string.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>