web/html/pkgsubmit.php: Revamp tarball validation

web/html/pkgsubmit.php: Revamp tarball validation

* Reorder checks.
* Use simple string functions instead of regular expressions.
* Check for type flags before validating paths.

The latter ensures we don't treat tarball keywords/flags as directories.
This avoids problems with bsdtar inserting PaxHeader attributes into the
archive which look something like the following to Archive_Tar:

    PaxHeader/xcursor-protozoa
    xcursor-protozoa/
    xcursor-protozoa/PaxHeader/PKGBUILD
    xcursor-protozoa/PKGBUILD

This only occurs on certain filesystems (e.g. jfs), but the tarball is
by no means invalid. When extracted, it will only contain the PKGBUILD
within a single subdirectory.

Addresses FS#28802.

Thanks-to: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>