posix: implement `uname()`

[ycsin]

There are currently 20 missing POSIX functions with trivial implementations, uname() is one of them.

uname - get the name of the current system

The uname() function shall store information identifying the current system in the structure pointed to by name.

The uname() function uses the utsname structure defined in <sys/utsname.h>.

The uname() function shall return a string naming the current system in the character array sysname. Similarly, nodename shall contain the name of this node within an implementation-defined communications network. The arrays release and version shall further identify the operating system. The array machine shall contain a name that identifies the hardware that the system is running on.

The format of each member is implementation-defined.

See https://pubs.opengroup.org/onlinepubs/9699919799/functions/uname.html

See RFC #51211 for more info.