Merge bitcoin/bitcoin#26078: p2p: return CSubNet in LookupSubNet

fb3e812277 p2p: return `CSubNet` in `LookupSubNet` (brunoerg)

Pull request description:

  Analyzing the usage of `LookupSubNet`, noticed that most cases uses check if the subnet is valid by calling `subnet.IsValid()`, and the boolean returned by `LookupSubNet` hasn't been used so much, see:
  29d540b7ad/src/httpserver.cpp (L172-L174)
  29d540b7ad/src/net_permissions.cpp (L114-L116)

  It makes sense to return `CSubNet` instead of `bool`.

ACKs for top commit:
  achow101:
    ACK fb3e812277
  vasild:
    ACK fb3e812277
  theStack:
    Code-review ACK fb3e812277
  stickies-v:
    Concept ACK, but Approach ~0 (for now). Reviewed the code (fb3e812277) and it all looks good to me.

Tree-SHA512: ba50d6bd5d58dfdbe1ce1faebd80dd8cf8c92ac53ef33519860b83399afffab482d5658cb6921b849d7a3df6d5cea911412850e08f3f4e27f7af510fbde4b254

src/netbase.h

@@ -227,11 +227,9 @@ CService LookupNumeric(const std::string& name, uint16_t portDefault = 0, DNSLoo
  * @param[in]  subnet_str  A string representation of a subnet of the form
  *                         `network address [ "/", ( CIDR-style suffix | netmask ) ]`
  *                         e.g. "2001:db8::/32", "192.0.2.0/255.255.255.0" or "8.8.8.8".
- * @param[out] subnet_out  Internal subnet representation, if parsable/resolvable
- *                         from `subnet_str`.
- * @returns whether the operation succeeded or not.
+ * @returns a CSubNet object (that may or may not be valid).
  */
-bool LookupSubNet(const std::string& subnet_str, CSubNet& subnet_out);
+CSubNet LookupSubNet(const std::string& subnet_str);
 
 /**
  * Create a TCP socket in the given address family.