libc: implement sethostname()

sethostname() sets the hostname in struct utsname
declared in uname.c
This patch also removes hostname.cc from libc/network
which is a duplicate implementation of gethostname() and
is not used during the build.

Fixes #353

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>