busybox unxz does not support --threads/-T option

set git=true if git is missing so all git commands are ignored. This
simplifies the code a bit

valid operators are AND OR WITH

I've forgot to add a patch file to the source variable in an APKBUILD,
altought I did add it to the sums variable.

The error message made it
seem that I've forgot to add the file to the source directory, which led
me to check if my build system was missing the files for some reason.

Only after reading the `abuild.in` file did I understood what happened.

Hopefully this change makes the message clearer and more helpful.

licenses will be checked against the license.lst file provided by
the spdx-licenses-list package when installed except when explicitly
disabled by the !spdx options flag.

abuild, as packaged in Alpine Linux, does not depend on git. But when
you use it without git, it will print out errors like the following:

/usr/bin/abuild: line 2554: git: not found

With this commit, it saves the git_path in the beginning (just like
abuild_path). Later in the code it does not try to run git if that
variable is empty.

Notably `abuild rootbld` is already checking whether `abuild-rootbld`
is installed, and that subpackage of `abuild` does already depend on
`git`. So no additional check was added before using `git ` inside
`rootbld`.

Fixes #32

The force flag used to skip the following functions, without any
documentation in the help (-h) output:

  * verify (checksum verification)
  * initdcheck (check if the init scripts are openrc scripts)
  * check_arch (check if the target architecture is in "arch=")
  * check_libc (check if the target libc is masked in the options)

This was counter-intuitive and could even be dangerous (when one relies
on the checksum verification to prevent man-in-the-middle attacks, but
always uses the -f flag).

With this commit, it only skips check_arch and check_libc besides the
package up to date check and the help output mentions this.