fetchmailrc split: ignore commented lines

[gutmensch]

* currently a commented line with "poll" in it is not ignored
  although [fetchmail source](https://gitlab.com/fetchmail/fetchmail/-/blob/legacy_64/rcfile_l.l#L218) defines
  hash as valid comment character

* this can cause the script to setup a broken service file, which
  results in supervisor indefinite restart loops and high cpu usage

* suggestion: delete all commented lines before parsing

Description

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[georglauterbach]

This PR will be merged after #2299 is merged.

Approximately tomorrow (29th) or the day(s) after.