config: bug: fix truncation of included config files on permissions error

ast_config_text_file_save() currently truncates include files as they
are processed.  If a subsequent include file or the main config file has
a permissions error that prevents writing, earlier include files are left
truncated resulting in a frantic search for backups.

This patch causes ast_config_text_file_save to check for write access
on all files before it truncates any of them.

Will be applied 1.8 > trunk.

Tested by: George Joseph
Review: https://reviewboard.asterisk.org/r/3986/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@422900 f38db490-d61c-443f-a65b-d21fe96a405b