Make some ECDSA failures more informative/accurate
Rename and simplify packing helper functions
[DNSSEC] Make int to bytes conversions fixed length in ECDSA

  ECDSA public keys consist of a single value, called "Q" in FIPS
  186-3.  In DNSSEC keys, Q is a simple bit string that represents the
  uncompressed form of a curve point, "x | y".

  The ECDSA signature is the combination of two non-negative integers,
  called "r" and "s" in FIPS 186-3.  The two integers, each of which is
  formatted as a simple octet string, are combined into a single longer
  octet string for DNSSEC as the concatenation "r | s".  (Conversion of
  the integers to bit strings is described in Section C.2 of FIPS
  186-3.)  For P-256, each integer MUST be encoded as 32 octets; for
  P-384, each integer MUST be encoded as 48 octets.
[DNSSEC] Make int to bytes conversions fixed length in DSA (RFC 2536)

This fixes DSA key generation.
Extend the fixed-length int representation fixes to sig0.go
Tests: wait for the server to be started before proceeding
Tests: silence a data race detected during server tests
Merge pull request #157 from FiloSottile/intermittent_errors

Fix some intermittent errors