fread: preserve literal "NA" header names

[shrektan]

Closes #5124

fread() currently reuses regular field NA parsing while reading header names, so a literal header value "NA" is treated as missing when na.strings includes "NA" and gets replaced with an auto-generated Vn name.

This patch makes header parsing preserve the literal column name while leaving normal data parsing unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.04%. Comparing base (9a8f664) to head (6b5cd8c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7686   +/-   ##
=======================================
  Coverage   99.04%   99.04%           
=======================================
  Files          87       87           
  Lines       17031    17037    +6     
=======================================
+ Hits        16868    16874    +6     
  Misses        163      163           

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[github-actions]

No obvious timing issues in HEAD=fix-5124

Generated via commit 6b5cd8c

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	5 minutes and 22 seconds
Installing different package versions	11 minutes and 53 seconds
Running and plotting the test cases	4 minutes and 1 seconds