Make it clear which functions that are intended to be translation unit local

Do not share functions that are meant to be translation unit local with
other translation units. Use internal linkage for those consistently.

src/test/test_bitcoin_fuzzy.cpp

@@ -52,7 +52,7 @@ enum TEST_ID {
     TEST_ID_END
 };
 
-bool read_stdin(std::vector<uint8_t> &data) {
+static bool read_stdin(std::vector<uint8_t> &data) {
     uint8_t buffer[1024];
     ssize_t length=0;
     while((length = read(STDIN_FILENO, buffer, 1024)) > 0) {
@@ -63,7 +63,7 @@ bool read_stdin(std::vector<uint8_t> &data) {
     return length==0;
 }
 
-int test_one_input(std::vector<uint8_t> buffer) {
+static int test_one_input(std::vector<uint8_t> buffer) {
     if (buffer.size() < sizeof(uint32_t)) return 0;
 
     uint32_t test_id = 0xffffffff;