[With hexadecimal color values now handled via their own routine,
re-introduce the negative unsigned numbers fix.]

strtoul() has a side effect that when given a string representing a
negative number, it treats it as a high value hexadecimal.  IOW,
strtoul("-42", &val) sets val to 0xffffffd6.  This could potentially
result in unintended surprise behaviors.

Catch this by using strtol() and then manually check for the negative
value.  This logic is modelled after Wayland's strtouint().

Note that this change unfortunately reduces the range of parseable
numbers from [0,UINT_MAX] to [0,INT_MAX].  The current users of
weston_config_section_get_uint() are anticipating numbers far smaller
than either of these limits, so the change is believed to have no impact
in practice.

Also add a test case for negative numbers that catches this error
condition.

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