strvec: rename files from argv-array to strvec

strvec: rename files from argv-array to strvec

This requires updating #include lines across the code-base, but that's
all fairly mechanical, and was done with:

  git ls-files '*.c' '*.h' |
  xargs perl -i -pe 's/argv-array.h/strvec.h/'

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>