Namelist defaults with line breaks in the namelist_defaults_*.xml files need to have special handling

### Brief summary of bug

The build-namelist explicitly removes newlines for settings when handled in test-mods or user-mods (in the user_nl_* files) or in CLM_NAMELIST_OPTS. However, it doesn't do this when newlines are added to the namelist_defaults XML files. In ctsm5.3.051 a new function was added to handle this for megan_specifier and drydep_list called remove_newlines.

This same issue **could** come up for other variables and possibly what really should happen is that this just always be done for all variables.

### General bug information

**CTSM version you are using:** ctsm5.3.051
**Does this bug cause significantly incorrect results in the model's science?** No

**Configurations affected:** namelist defaults with values that span more than one line of text

### Details of bug

See the comment here that came up when @slevis-lmwg and I discussed this:

https://github.com/ESCOMP/CTSM/pull/3125/files#r2114959780