net: socketutils: Add utils to manipulate network address strings

Two utils to manipulate addresses in format "addr[:port]". I.e.,
network address (domain name or numeric), optionally followed by
port number:

* net_addr_str_find_port(), to return pointer to port number
substring (or NULL if not present).
* net_getaddrinfo_addr_str(), which is effectively getaddrinfo()
wrapper taking a "addr[:port]" string as a parameter.

The header file is named socketutils.h to emphasize that these
utility functions are implemented on top of BSD Sockets API
(and other POSIX/ANSI C functions), and thus portable to other
POSIX systems (e.g., Linux), so can be used in apps testing
POSIX compatibility. More utility functions (beyond address
manipulation) can be added later.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>