Fix truncated pattern space problem

[LoukasPap]

Fix issue 253

• Initialize the context of the hold space to \n (newline character)
• Previously, it started as an empty string which resulted to wrong outputs

This solution not only fixes the problem for the swap pattern x but also for g and G .

[LoukasPap]

By mistake, I contained commits from other PRs in this PR. I'll try to fix it.

[github-actions]

GNU sed testsuite comparison:

Test results comparison:
  Current:   TOTAL: 64 / PASSED: 8 / FAILED: 56 / SKIPPED: 0
  Reference: TOTAL: 64 / PASSED: 8 / FAILED: 56 / SKIPPED: 0

[LoukasPap]

Creating a new PR with only the commits needed.