[cmyth] Fix signed/unsigned comparison warnings

file.c: In function 'cmyth_file_seek':
file.c:511:8: warning: comparison between signed and unsigned integer
    expressions [-Wsign-compare]
  if(c > sizeof(msg))
       ^

socket.c: In function '__cmyth_toupper_string':
socket.c:2996:17: warning: comparison between signed and unsigned
    integer expressions [-Wsign-compare]
  for ( i=0 ; i < sizeof(str) && str[i] != '\0' ; i++ ) {
                ^
[cmyth] Fix variable set but not used warnings

mythtv_mysql.c: In function 'cmyth_mysql_get_recording_seek_offset':
mythtv_mysql.c:2323:10: warning: variable 'prev_mark' set but not used
    [-Wunused-but-set-variable]
 int64_t prev_mark = 0;
         ^

mythtv_mysql.c:2321:10: warning: variable 'next_mark' set but not used
    [-Wunused-but-set-variable]
  int64_t next_mark = 0;
          ^