journald: do not store the iovec entry for process commandline on stack

This fixes a crash where we would read the commandline, whose length is under
control of the sending program, and then crash when trying to create a stack
allocation for it.

CVE-2018-16864
https://bugzilla.redhat.com/show_bug.cgi?id=1653855

The message actually doesn't get written to disk, because
journal_file_append_entry() returns -E2BIG.

src/basic/io-util.h

@@ -71,3 +71,5 @@ static inline bool FILE_SIZE_VALID_OR_INFINITY(uint64_t l) {
 #define IOVEC_MAKE(base, len) (struct iovec) IOVEC_INIT(base, len)
 #define IOVEC_INIT_STRING(string) IOVEC_INIT((char*) string, strlen(string))
 #define IOVEC_MAKE_STRING(string) (struct iovec) IOVEC_INIT_STRING(string)
+
+char* set_iovec_string_field(struct iovec *iovec, size_t *n_iovec, const char *field, const char *value);