reduce heap abuse in archive_fgets

Allocate each archive_read_buffer in load_repo with a static line size,
rather than reallocating for each package. Based on this, we can remove
much of the complexity in archive_fgets. Results in a small, but
noticeable boost in performance.
cleanup unnecessary assignments
avoid useless cast
include recommended TCMALLOC_CFLAGS
discourage use of DEFS, always use AM_CPPFLAGS
whitespace police
pkgfile: simplify search_single_repo

No need to allocate memory here, just split the (mutable) string.
create archive_conv struct for repo repacking

The prototype for write_cpio_entry was looking kind of awful, and I
wanted to pass the repo name for better errors. Instead of adding yet
another param, just glom it all into a struct for convenience.
separate out archive open/close logic to separate functions
stack allocate package "name" field
remove useless compiler flags from configure.ac
prepare v6
fix compiler warning on i686
update: fix grammatical error in child wait messaging

"waiting for" is more proper than "waiting on", as suggested by an
anonymous email and wait(2).
simplify return values from filefunc
simplify checking of select(2) return

Meh, this code is going away soon anyways.
update: reinitialize data buffer on download fail

Prevents a sequence such as the one seen below:

:: Updating 10 repos...
  download complete: allanbrokeit         [  1308.0   B  5.27K/s  9 remaining]
warning: download failed: http://example.com/foo/os/x86_64/foo.files [HTTP 404]
  download complete: foo                  [    22.6 KiB   186K/s  8 remaining]
error: failed to create archive reader for foo: Unrecognized archive format
  download complete: staging              [   101.9 KiB   156K/s  7 remaining]
update: avoid active_handle "reffing"

This serves no purpose. Just continue the loop and active_handles will
be given a new value on the next iteration.
remove unneeded cast
remove return value from curl_multi loop

We never check this, nor do we particularly care. Return values are
aggregated off of the easy handles. In the case of catastrophic failure,
there's probably something we're missing out on, but I'll lazily deal
with it if/when it happens.