The argv length calculation was not only off by one, but failed to take

The argv length calculation was not only off by one, but failed to take
into account the extra space required by expanded quotes, backslashes and
newlines.  Instead of pre-allocating a (possibly too short) buffer, start
with a 64-byte buffer and double it every time we come close to filling
it up.  Also, avoid appending a trailing space before the final newline.

This issue was uncovered by Kristoffer Gleditsch <kristoffer@linpro.no>,
who also helped test this patch.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1287 d4fa192b-c00b-0410-8231-f00ffab90ce4