feature(uname): Add -o/--operating-system flag
feature(uname): Add -o/--operating-system flag

The code for the flag already existed it had just not been added to the argument parsing. Replace "print the operating system name." in -s help text with "print the kernel name." so the -o option can use the previous -s help text.

uutils/coreutils,uutils/coreutils,uutils/coreutils