Performance Bug Fixing

1. Two useless memset
  1.1 Download.c:Line 110
    memset(rbuf, GETRECVSIZ, 0)
  1.2 Misc.c:Line 149
    memset(str, 0, size)
2. Unused/cacheable function result
  2.1 Aget.c:Line 93,95
    calc_offset
  2.2 Aget.c:Line 97,98
    inet_addr, htons
3. Unused initializing
  3.1 Aget.c:Line 21
    s = url
  3.2 Signal.c:Line 23
    arg = NULL
Merge pull request #2 from james0zan/fix-perf-bug

Performance Bug Fixing