config-parser: Improve error checks for strtol/strtoul calls

config-parser: Improve error checks for strtol/strtoul calls

Check errno, which is set of over/underflow, out of range, etc.  Also
check for empty strings (the usages covered in this patch already also
cover the case where there are non-digits present).  Set errno to 0
before making the strto*l call in case of pre-existing errors
(i.e. ENOTTY when running under the testsuite).

This follows the error checking style used in Wayland
(c.f. wayland-client.c and scanner.c).

In tests, also check errno, and add testcases for parsing '0'.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>