Move util/url to common/url.
Also add missing event_* flags to libbitcoin_util. #26293 + the commit dropping boost cppflags from libbitcoin_util shows this issue. i.e:
0 CXX util/libbitcoin_util_a-url.o
1util/url.cpp:7:10: fatal error: 'event2/http.h' file not found
2#include <event2/http.h>
3 ^~~~~~~~~~~~~~~
41 error generated.