su CLI overhaul

[japaric]

this PR:

• refactors su's command line parsing to avoid panicky slicing operations
• fixes su assumes that args after the first positional argument are all positional arguments #790
• fixes su parses '-g ' ghost as -g ghost #791
• adds support for the -- separator
• makes the CLI more strict, namely:
  • repeated boolean flags are rejected, e.g. su -l -l is rejected
  • flags unrelated to a working mode are rejected, e.g. su --version root is rejected

this PR is best reviewed commit by commit

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (870d15b) 55.88% compared to head (bde50e8) 57.44%.
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #792      +/-   ##
==========================================
+ Coverage   55.88%   57.44%   +1.55%     
==========================================
  Files          74       74              
  Lines        9968    10523     +555     
==========================================
+ Hits         5571     6045     +474     
- Misses       4397     4478      +81     

Files	Coverage Δ
src/su/context.rs	61.11% <100.00%> (+0.65%)	⬆️
src/su/mod.rs	0.00% <0.00%> (ø)
src/su/cli.rs	94.63% <92.76%> (+1.89%)	⬆️

... and 17 files with indirect coverage changes

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[pvdrz]

I saw this IsAbsent trait that was also on the sudo CLI overhaul. Could those two be shared? Maybe they are but I overlooked it.

[japaric]

I saw this IsAbsent trait that was also on the sudo CLI overhaul. Could those two be shared? Maybe they are but I overlooked it.

that should be done in a follow up PR. I didn't base the su refactor on top of the sudo refactor branch so I couldn't reuse the trait.