Problem:
On Windows, "gf" fails on a filepath that has a line:column suffix.
Example:
E447: Can't find file "src/app/core/services/identity/identity.service.ts:64:23"
Solution:
- Remove ":" from 'isfname' on Windows. Colon is not a valid filename
character (except for the drive-letter).
- Handle drive letters specially in file_name_in_line().
Fixes#25160
Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.
