Replace ~ for ignoring outputs with "ignored" variables.

This syntax was introduced in R2009b, so we cannot use it if we want to
support R2009a.
Backwards compatibility fixes for R2009a.

 - Use quadgk() instead of integral() for old MATLAB versions when doing
   conv() the brute force way.
 - Use a wrapper function to vectorize nextpow2() when needed.
 - Use seedRNG() in several chebfun2 tests instead of rng(), which is
   newer syntax.
Fix bug in inv() with "roots" algorithm.

We want "temp" to be the root corresponding to the minimum error here,
not the value of the minimum error itself.
Minor fixes to tests to get them to pass in R2009a.

Most of these are tolerance tweaks.  Replaced use of random exponents in
tests/singfun/test_constructor.m with fixed, arbitrary ones.  (The
R2009a RNG generates different numberse, and these numbers, and some of
them happen to be ones with which the singularity detector has trouble.)
Merge pull request #864 from chebfun/fix-r2009a

fix-r2009a