Revert "removed unused-result warnings"

Removing -Wunused-result on a few platforms caused only more
-Wunused-variable warnings on all platforms.

According to gcc documentation the attribute "warn_unused_result" is
useful for functions where not checking the result is either a security
problem or always a bug.

See https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#Function-Attributes

This reverts commit 9acb5f1edb8f73c97bdcd23877bace88e8421eca.

Conflicts:
	src/libYARP_OS/include/yarp/os/impl/RunProcManager.h
	src/libYARP_OS/include/yarp/os/impl/RunReadWrite.h