This PR fixes critical issues with XMPP stream parsing and adds a new helper function for MUC testing.
## Changes
### 1. Fix XMPP stream parsing for multiple stanzas
The XML parser was not correctly handling multiple consecutive stanzas in the stream. Fixed depth tracking to properly detect stanza boundaries.
### 2. Fix XMPP stream parsing depth tracking
Added proper depth tracking for nested XML elements to correctly identify when a complete stanza has been received.
### 3. Remove debug logging that interfered with expect tests
Removed verbose debug output that was causing expect-based functional tests to fail due to unexpected output in the stream.
### 4. Add `stbbr_for_presence_to` for MUC join presence matching
New helper function that registers a canned response for presence stanzas based on the `to` attribute. This is essential for testing MUC (Multi-User Chat) join flows where the response must be triggered by presence to a specific room JID.
**API:**
```c
void stbbr_for_presence_to(const char *to, const char *response);
Reviewed-on: jabber.developer2/stabber#1
Co-authored-by: jabber.developer2 <jabber.developer2@jabber.space>
Co-committed-by: jabber.developer2 <jabber.developer2@jabber.space>
Profanity Travis CI showed that stabber doesn't build anymore on Debian
Testing, Arch Linux and Fedora.
It appears all these systems have updated to libmicrohttpd 0.9.71
recently.
Earlier versions build fine.
See release announcement: https://lists.gnu.org/archive/html/libmicrohttpd/2020-06/msg00013.html
```
Furthermore, the release introduces an 'enum MHD_Result' instead of
for certain API misuse bugs by providing better types (not everything is
an 'int'). While this does NOT change the binary API, this change
_will_ cause compiler warnings for all legacy code -- until 'int' is
replaced with 'enum MHD_Result'.
```
