Improve FortiManager OS support

[JSCU-CNI]

This PR improves FortiManager OS support in dissect.

[codecov]

Codecov Report

Attention: Patch coverage is 41.07143% with 33 lines in your changes missing coverage. Please review.

Project coverage is 77.02%. Comparing base (88fe795) to head (2c8cd33).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...issect/target/plugins/os/unix/linux/fortios/_os.py	17.94%	32 Missing ⚠️
dissect/target/plugins/os/windows/_os.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #921      +/-   ##
==========================================
+ Coverage   76.98%   77.02%   +0.03%     
==========================================
  Files         322      322              
  Lines       27573    27575       +2     
==========================================
+ Hits        21228    21239      +11     
+ Misses       6345     6336       -9     

Flag	Coverage Δ
unittests	77.02% <41.07%> (+0.03%)	⬆️

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