lib/config: use g_key_file_get_string to read config values

lib/config: use g_key_file_get_string to read config values

Unlike the previous g_key_file_get_value, this version processes
escape codes for whitespace and \. The remaining two broken tests from
the last commit are because "notmuch config get" treats every value as
a list, and thus the previously introduces stripping of leading
whitespace applies.