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.h

@@ -116,7 +116,8 @@ const char* split(const char **state, size_t *l, const char *separator, bool quo
 char *strappend(const char *s, const char *suffix);
 char *strnappend(const char *s, const char *suffix, size_t length);
 
-char *strjoin(const char *x, ...) _sentinel_;
+char *strjoin_real(const char *x, ...) _sentinel_;
+#define strjoin(a, ...) strjoin_real((a), __VA_ARGS__, NULL)
 
 #define strjoina(a, ...)                                                \
         ({                                                              \