adding PKGBUILD.in
default make target is `most`
the shebang says zsh

this should simplify support for multiple package descriptions
such as PKGBUILD files (ArchLinux packages).

the absence of arrays in sh makes dealing with multiple values
(think `extract $tarball $paths`) difficult and awkward; bash
adds too little value for the price of leaving the POSIX subset.

implementation notes:

* nonomatch, nonullglob (NO_NOMATCH, NO_NULL_GLOB) behaviors
  are expected by both code and tests.
* zsh does not run the EXIT trap when bailing out through ERR_EXIT,
  it does run the ZERR trap.
* exithook now normalizes the exitcode to [0, 1] since zsh passes
  the actual exit code of the failed command to the ZERR trap.
  this showed up in tests/580-tarball-hook-failure: this test
  defines a bsu_tarball_hook which returns 3, yet in sh the trap
  code received 1 in $?.
* function body-trailing redirections are not special in zsh,
  they're active while the function statement is executed,
  not when the function runs.