Split url.c into url_curl.c (curl implementation) and url_unreal

(the new fallback https-only implementation).

./configure will set URL= to either url_curl.o or url_unreal.o
depending on whether curl is enabled or not.

The 3 functions that both implementations had in common are now in
src/misc.c: url_is_valid(), displayurl() and url_getfilename().

Makefile.in

@@ -90,6 +90,7 @@ XCFLAGS=@PTHREAD_CFLAGS@ @PCRE2_CFLAGS@ @ARGON2_CFLAGS@ @CARES_CFLAGS@ @SODIUM_C
 IRCDMODE = 711
 
 # Objects that are optional due to optional libraries:
+URL=@URL@
 GEOIP_CLASSIC_OBJECTS=@GEOIP_CLASSIC_OBJECTS@
 GEOIP_CLASSIC_LIBS=@GEOIP_CLASSIC_LIBS@
 GEOIP_CLASSIC_CFLAGS=@GEOIP_CLASSIC_CFLAGS@
@@ -119,6 +120,7 @@ MAKEARGS =	'CFLAGS=${CFLAGS}' 'CC=${CC}' 'IRCDLIBS=${IRCDLIBS}' \
 		'SHELL=${SHELL}' \
 		'CRYPTOLIB=${CRYPTOLIB}' \
 		'CRYPTOINCLUDES=${CRYPTOINCLUDES}' \
+		'URL=${URL}' \
 		'GEOIP_CLASSIC_OBJECTS=${GEOIP_CLASSIC_OBJECTS}' \
 		'GEOIP_CLASSIC_LIBS=${GEOIP_CLASSIC_LIBS}' \
 		'GEOIP_CLASSIC_CFLAGS=${GEOIP_CLASSIC_CFLAGS}'