conf-parser: accept trailing backslash at the end of the file (#8941)

conf-parser: accept trailing backslash at the end of the file (#8941)

This makes it behave the same whether there is a blank line or not at
the end of the file.  This is also consistent with the behavior of the
shell on a shell script that ends on a trailing backslash at the last
line.

Added tests to test_config_parse(), which only pass if the corresponding
change to config_parse() is included.