util-lib: optionally, when writing a string to a file, verify string on failure

With this change, the idiom:

    r = write_string_file(p, buf, 0);
    if (r < 0) {
           if (verify_one_line_file(p, buf) > 0)
                   r = 0;
    }

gets reduced to:

    r = write_string_file(p, buf, WRITE_STRING_FILE_VERIFY_ON_FAILURE);

i.e. when writing the string fails and the new flag
WRITE_STRING_FILE_VERIFY_ON_FAILURE is specified we'll not return a
failure immediately, but check the contents of the file. If it matches
what we wanted to write we suppress the error and exit cleanly.

src/basic/fileio.h

@@ -34,6 +34,7 @@ typedef enum {
         WRITE_STRING_FILE_CREATE = 1,
         WRITE_STRING_FILE_ATOMIC = 2,
         WRITE_STRING_FILE_AVOID_NEWLINE = 4,
+        WRITE_STRING_FILE_VERIFY_ON_FAILURE = 8,
 } WriteStringFileFlags;
 
 int write_string_stream(FILE *f, const char *line, bool enforce_newline);
@@ -43,7 +44,7 @@ int read_one_line_file(const char *fn, char **line);
 int read_full_file(const char *fn, char **contents, size_t *size);
 int read_full_stream(FILE *f, char **contents, size_t *size);
 
-int verify_one_line_file(const char *fn, const char *line);
+int verify_file(const char *fn, const char *blob, bool accept_extra_nl);
 
 int parse_env_file(const char *fname, const char *separator, ...) _sentinel_;
 int load_env_file(FILE *f, const char *fname, const char *separator, char ***l);