var: Use 16-bit container for type

[jlucovsky]

Continuation of #12076

Issue: 6855: Match sigmatch type field in var and bit structs

Link to ticket: https://redmine.openinfosecfoundation.org/issues/6855

Describe changes:

• Increase type in flowbit, flowvar and generic var
• Ensure type, idx, and next pointers align on each struct.

Updates:

• Modified FlowVarThreshold_ (missing from first PR)

Provide values to any of the below to override the defaults.

• To use an LibHTP, Suricata-Verify or Suricata-Update pull request,
  link to the pull request in the respective _BRANCH variable.
• Leave unused overrides blank or remove.

SV_REPO=
SV_BRANCH=
SU_REPO=
SU_BRANCH=
LIBHTP_REPO=
LIBHTP_BRANCH=

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.21%. Comparing base (a9b36d8) to head (c1beda3).
Report is 26 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12253      +/-   ##
==========================================
+ Coverage   83.18%   83.21%   +0.03%     
==========================================
  Files         912      912              
  Lines      257174   257174              
==========================================
+ Hits       213930   214011      +81     
+ Misses      43244    43163      -81     

Flag	Coverage Δ
fuzzcorpus	61.06% <ø> (+0.05%)	⬆️
livemode	19.41% <ø> (ø)
pcap	44.39% <ø> (-0.01%)	⬇️
suricata-verify	62.79% <ø> (+<0.01%)	⬆️
unittests	59.18% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[suricata-qa]

Information: QA ran without warnings.

Pipeline 23766

[inashivb]

Minor: There's another TODO for dealing w this issue in detect-engine-register.h. Could we remove that too with this work?

[jlucovsky]

Continued in #12272