res_phreaknet: iax.conf, et. al. are truncated when there is no disk space left

[InterLinked1]

If there is no disk space left, and res_phreaknet attempts to update IAX config files, it will be unable to successfully write any data and as a result, all the IAX config files are truncated to 0 bytes when they are updated.

A simple fix would be to detect the low disk space condition (< 1MB) and not attempt to do any file I/O in this situation. However, this could be subject to race conditions and doesn't feel like a completely proper fix for this.

Because we're using ast_config_text_file_save2, we can't directly handle this scenario in this module, such as by writing to a temp file first, ensuring we wrote everything we meant to write, and then renaming the file, at least without working around it in some other way.