cleanup: remove superfluous slash in default cache dir path
config: use small local static buffer instead of mempool for temporary config path

We can't use mempool for temporary variable for configuration path inside
find_config_tree_* functions since these functions can use the mempool
themselves deeper in the code and we can free mempool chunks only from
top to bottom which is not the case here (some default string
configuration values can be allocated from the mempool).
config: keep config paths with variable names properly marked on output