shared/curl-util: load libcurl via dlopen

shared/curl-util: load libcurl via dlopen

Convert curl-util to the dlopen pattern used by other optional shared
libraries in libshared (libarchive, pcre2, idn, ...). Declare the curl
API entry points with DLSYM_PROTOTYPE, resolve them in a dlopen_curl()
helper, and call the sym_* wrappers from callers. curl_glue_new() now
loads the library on first use, so consumers going through CurlGlue
pick this up automatically; journal-upload and report-upload call
dlopen_curl() directly since they use curl without the glue layer.

With this in place curl-util can live in libshared itself, linked only
against libcurl's headers (via libcurl_cflags). The libcurlutil_static
convenience library and the libcurl link dependency on systemd-imdsd,
systemd-pull, systemd-journal-upload and systemd-report go away.

Also move the easy_setopt() helper macro next to the DLSYM declarations
so all consumers use a single sym-prefixed definition, and add a
dlopen_curl() check to test-dlopen-so.