linux: add setsockopt(2) level constants
linux: add setsockopt(2) level constants

Although setsockopt(2) itself is part of POSIX, the
level constants are not.

tjol/vala-tjol,flobrosch/vala-fixes,frida/vala,playya/vala,Distrotech/vala,rvu95/vala,flobrosch/vala-fixes,frida/vala,spidermonkdust/vala,GNOME/vala,frida/vala,frida/vala,spidermonkdust/vala,rvu95/vala,jessevdk/vala,Distrotech/vala,severinh/vala,kamanashisroy/aroop,frida/vala,GNOME/vala,severinh/vala,rvu95/vala,playya/vala,severinh/vala,flobrosch/vala-fixes,tjol/vala-tjol,GNOME/vala,tjol/vala-tjol,kamanashisroy/aroop,xdave/pvala,GNOME/vala,jessevdk/vala,playya/vala,jessevdk/vala,Distrotech/vala,kamanashisroy/aroop,spidermonkdust/vala