tool: fix memory mixups

tool: fix memory mixups

memory allocated by libcurl must be freed with curl_free() and vice versa,
memory allocated by the tool itself must be freed with curlx_free().

- dynbuf: free libcurl data with curl_free()
- tool_operate: make sure we get URL using the right memory
- tool_operhlp: free libcurl memory with curl_free()
- tool_operate: free curl_maprintf() pointer with curl_free
- var: data from curlx_base64_decode needs curlx_free
- tool_operate: fix memory juggling in etag handling
- tool_cb_hdr: fix memory area mixups
- tool_operate: another mixup in etag management
- tool_cb_hdr: more memory mixup fixes
- tool_cfgable.c: document some details
- tool_help: show global-mem-debug in -V output

Closes #21099