fix multiple grounded dropship issues

[NickAragua]

Four changes:

• grounded dropships were unavailable as potential scenario modifiers even though they should have been
• customize scenario dialog would reset status to null if editing a non-resolved scenario
• invoke full "land" code to set all appropriate values when initializing a grounded aerospace unit
• don't deploy opfor directly underneath allied dropship in 'Defend Dropship' scenario

[codecov]

Codecov Report

Merging #2599 (44af08f) into master (7bfeedc) will increase coverage by 0.00%.
The diff coverage is 0.00%.

❗ Current head 44af08f differs from pull request most recent head 8010520. Consider uploading reports for the commit 8010520 to get more accurate results

@@            Coverage Diff            @@
##             master    #2599   +/-   ##
=========================================
  Coverage     10.73%   10.74%           
- Complexity     3804     3807    +3     
=========================================
  Files           700      700           
  Lines         97888    97891    +3     
  Branches      16305    16307    +2     
=========================================
+ Hits          10510    10516    +6     
  Misses        85999    85999           
+ Partials       1379     1376    -3     

Impacted Files	Coverage Δ	Complexity Δ
...hq/campaign/mission/AtBDynamicScenarioFactory.java	1.73% <0.00%> (-0.01%)	12.00 <0.00> (ø)
.../src/mekhq/gui/dialog/CustomizeScenarioDialog.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ekhq/campaign/universe/RandomFactionGenerator.java	58.80% <0.00%> (ø)	66.00% <0.00%> (ø%)
MekHQ/src/mekhq/campaign/mission/AtBContract.java	19.19% <0.00%> (+0.11%)	67.00% <0.00%> (+1.00%)
MekHQ/src/mekhq/campaign/universe/Planet.java	26.30% <0.00%> (+0.22%)	25.00% <0.00%> (ø%)
...ekHQ/src/mekhq/campaign/market/ContractMarket.java	58.77% <0.00%> (+0.39%)	113.00% <0.00%> (+1.00%)
MekHQ/src/mekhq/campaign/universe/Systems.java	42.23% <0.00%> (+0.79%)	36.00% <0.00%> (+1.00%)

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