naemon/utils.c: treating correctly pipe() return value

if pipe fails, we log a message and return UNKNOWN

Signed-off-by: Ricardo Maraschini <ricardo.maraschini@gmail.com>
naemon/utils.c: my_system_r() - removing not needed else statement

making code easier to read with less indentation

Signed-off-by: Ricardo Maraschini <ricardo.maraschini@gmail.com>
naemon/utils.c: avoiding unnecessary comparsions

if our call to read succeeded we may be able to
keep it going on without further validations/comparsions.

Signed-off-by: Ricardo Maraschini <ricardo.maraschini@gmail.com>