stage1: handle parsing errors in systemd version
stage1: handle parsing errors in systemd version

If we can't parse a string of the form v229, assume the version in the
"systemd-version" file is a branch name and indicate callers the version
couldn't be guessed.

In that case, assume it's new enough for "systemd exit" to work.

joshix/rkt,joshix/rkt,euank/rkt,Wilybald/rkt,yifan-gu/rkt,Wilybald/rkt,joshix/rkt,yifan-gu/rkt,akshaykarle/rkt,sstabellini/rkt,lucasallan/rkt,coreos/rkt,tmrts/rkt,alban/rkt,Shashwatsh/rkt,coreos/rkt,jakon89/rkt,sstabellini/rkt,yifan-gu/rocket,jjlakis/rkt,jjlakis/rkt,euank/rkt,markllama/rkt,achanda/rkt,il9ue/rkt,s-urbaniak/rkt,sgotti/rkt,akshaykarle/rkt,cab105/rkt,nhlfr/rkt,monstermunchkin/rkt,lucasallan/rkt,dgonyeo/rkt,akshaykarle/rkt,kinvolk/rkt,markllama/rkt,lucasallan/rkt,jakon89/rkt,jonboulle/rkt,sgotti/rkt,kinvolk/rkt,rkt/rkt,ybubnov/rkt,dgonyeo/rkt,sgotti/rkt,shobhit85/rkt,squeed/rkt,sstabellini/rkt,jonboulle/rkt,il9ue/rkt,Wilybald/rkt,s-urbaniak/rkt,kinvolk/rkt,matthaias/rkt,mike1729/rkt,rkt/rkt,alban/rkt,squeed/rkt,euank/rkt,shobhit85/rkt,ybubnov/rkt,monstermunchkin/rkt,Shashwatsh/rkt,nhlfr/rkt,rkt/rkt,dgonyeo/rkt,il9ue/rkt,tmrts/rkt,coreos/rkt,joshix/rkt,achanda/rkt,lucasallan/rkt,tmrts/rkt,jjlakis/rkt,shobhit85/rkt,markllama/rkt,nhlfr/rkt,squeed/rkt,Wilybald/rkt,yifan-gu/rocket,monstermunchkin/rkt,jonboulle/rkt,intelsdi-x/rkt,mike1729/rkt,intelsdi-x/rkt,alban/rkt,matthaias/rkt,jakon89/rkt,s-urbaniak/rkt,intelsdi-x/rkt,Shashwatsh/rkt,rkt/rkt,ybubnov/rkt,yifan-gu/rkt,cab105/rkt,achanda/rkt,cab105/rkt,mike1729/rkt,yifan-gu/rocket,kinvolk/rkt,matthaias/rkt