src/vipw.c: Some readability improvements

[alejandro-colomar]

I find this file quite hard to read. This improves the situation a little bit, although more follow-up work is needed.

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Revisions:

v1b

• Rebase

$ git rd 
1:  a8424d5fea62 = 1:  da6671ec2a5d src/vipw.c: vipwedit(): Reduce scope of local variable
2:  141b7078a631 = 2:  4154cf22ee7d src/vipw.c: vipwexit(), usage(): Mark as [[noreturn]]
3:  fead1f4fc4c9 = 3:  eda0b4f6fb45 src/vipw.c: vipwedit(): Don't else after [[noreturn]]
4:  b34974d7e7cf = 4:  3d311a157cb1 src/vipw.c: vipwedit(): Set fileedit closer to its use