tree-wide: drop NULL sentinel from strjoin

This makes strjoin and strjoina more similar and avoids the useless final
argument.

spatch -I . -I ./src -I ./src/basic -I ./src/basic -I ./src/shared -I ./src/shared -I ./src/network -I ./src/locale -I ./src/login -I ./src/journal -I ./src/journal -I ./src/timedate -I ./src/timesync -I ./src/nspawn -I ./src/resolve -I ./src/resolve -I ./src/systemd -I ./src/core -I ./src/core -I ./src/libudev -I ./src/udev -I ./src/udev/net -I ./src/udev -I ./src/libsystemd/sd-bus -I ./src/libsystemd/sd-event -I ./src/libsystemd/sd-login -I ./src/libsystemd/sd-netlink -I ./src/libsystemd/sd-network -I ./src/libsystemd/sd-hwdb -I ./src/libsystemd/sd-device -I ./src/libsystemd/sd-id128 -I ./src/libsystemd-network --sp-file coccinelle/strjoin.cocci --in-place $(git ls-files src/*.c)

git grep -e '\bstrjoin\b.*NULL' -l|xargs sed -i -r 's/strjoin\((.*), NULL\)/strjoin(\1)/'

This might have missed a few cases (spatch has a really hard time dealing
with _cleanup_ macros), but that's no big issue, they can always be fixed
later.

src/basic/string-util.c

@@ -218,7 +218,7 @@ char *strappend(const char *s, const char *suffix) {
         return strnappend(s, suffix, suffix ? strlen(suffix) : 0);
 }
 
-char *strjoin(const char *x, ...) {
+char *strjoin_real(const char *x, ...) {
         va_list ap;
         size_t l;
         char *r, *p;