cgi-io: iron out extra compiler warnings

cgi-io: iron out extra compiler warnings

Fixes following errors:

 main.c:458:37: error: comparison of integer expressions of different signedness: ‘int’ and ‘size_t’ {aka ‘long unsigned int’} [-Werror=sign-compare]
 main.c:463:17: error: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Werror=sign-compare]
 main.c:518:35: error: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘long int’} and ‘size_t’ {aka ‘long unsigned int’} [-Werror=sign-compare]
 main.c:157:3: error: ignoring return value of ‘read’, declared with attribute warn_unused_result [-Werror=unused-result]
 main.c:763:3: error: ignoring return value of ‘chdir’, declared with attribute warn_unused_result [-Werror=unused-result]

Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit bb6cdb804cc4db12cca776f559baa6d989a992ec)