posix: Fix FILE bindings (copy from GLib.FileStream), and add popen.
posix: Fix FILE bindings (copy from GLib.FileStream), and add popen.

Fixes bug 595880.

jessevdk/vala,kamanashisroy/aroop,spidermonkdust/vala,flobrosch/vala-fixes,frida/vala,GNOME/vala,Distrotech/vala,frida/vala,phako/vala,frida/vala,Jc2k/vala,rvu95/vala,severinh/vala,rvu95/vala,playya/vala,severinh/vala,themiwi/vala,GNOME/vala,xdave/pvala,spidermonkdust/vala,kamanashisroy/aroop,severinh/vala,Jc2k/vala,GNOME/vala,flobrosch/vala-fixes,jessevdk/vala,tjol/vala-tjol,themiwi/vala,jessevdk/vala,kamanashisroy/aroop,spidermonkdust/vala,Distrotech/vala,phako/vala,frida/vala,themiwi/vala,playya/vala,GNOME/vala,frida/vala,rvu95/vala,flobrosch/vala-fixes,tjol/vala-tjol,playya/vala,Distrotech/vala,tjol/vala-tjol