APKs have been created with GNU tar so far, which uses the
GNU extensions for long names. In order to increase portability
support the standard header's 'prefix' portion in case
the GNU extensions are not present.

Depending how the directory entries are ordered, the cached dir
instance might not have been updated correctly. This has not been
a problem as the entries have been ordered, but is now triggered
on ppc.

The main usage is for alpine-chroot-install and similar scripts, so it
can depend on specific *persistent* version.

This reduces function pointers in heap, and unifies how the
io functions are called.

Detect properly if the file stream gets an error during these
read operations.

Reported-by: Ariel Zelivansky from Twistlock

The value from tar header is unsigned int; keep it casted to
unsigned int and size_t instead of (signed) int, otherwise
the comparisons fail to do their job properly. Additionally check
entry.size against SSIZE_MAX so the rounding up later on is
guaranteed to not overflow.

Fixes CVE-2017-9669 and CVE-2017-9671.
Reported-by: Ariel Zelivansky from Twistlock

This reverts commit ee5ce728.

This behaviour has been changed in: 7d5cd2c8.

Variable QEMU_EMULATOR is set by enter-chroot script from
alpine-chroot-install.

- rework the progress bar a little bit, basically removing the [ and ] to give a more
  modern aesthetic.

- if utf-8 locale is enabled, use unicode codepoint 0x2588 instead of # to give the
  progress bar a nicer look.

- if APK_PROGRESS_CHAR environment variable is defined, use the character defined there
  for the progress bar.

This is useful for requirements such as: python3=~3.6, which would match python3-3.6.[0-9].
This implementation should in theory be backwards compatible with the implementation in Adelie.

Introduced in commit c0f2d88f.
fstatfs is needed to inspect the mount flags.