xbps_entry_install_conf_file: add more assertions.
libxbps: two new functions to get the pkgver and arch components in a filename.

- xbps_binpkg_pkgver() returns a heap allocated string with the pkgver component.
- xbps_binpkg_arch() returns a heap allocated string with the architecture component.

A basename, full path or relative path is supported, i.e:

	/path/to/foo-1.0_1.x86_64.xbps
	../../foo/blah-0.1_1.x86_64.xbps
	baz-0.1_1.x86_64.xbps
xbps-uhelper: new targets: binpkgarch and binpkgver.

- binpkgarch: returns the architecture component of a xbps filename.
- binpkgver: returns the pkgver component of a xbps filename.
xbps-rindex: use xbps_binpkg_{arch,pkgver} for -r (remove-obsoletes).

This is magnituds faster than before; some results:

$ for f in 1 2 3; do time xbps-rindex -r /var/cache/xbps/ &>/dev/null; done

real 0m0.624s
user 0m2.163s
sys 0m0.032s

real 0m0.590s
user 0m2.159s
sys 0m0.023s

real 0m0.584s
user 0m2.144s
sys 0m0.039s

$ for f in 1 2 3; do time LD_PRELOAD=$PWD/lib/libxbps.so.2.0.0 ./bin/xbps-rindex/xbps-rindex -r /var/cache/xbps &>/dev/null; done

real 0m0.037s
user 0m0.030s
sys 0m0.010s

real 0m0.036s
user 0m0.032s
sys 0m0.007s

real 0m0.037s
user 0m0.035s
sys 0m0.006s
$
xbps-remove: use xbps_binpkg_{arch,pkgver} for -O (clean-cache).

The new implementation is almost 1s faster than before on my system.