* prevent the script form blindly overwriting the current configuration
* force maddyctl linking so that we can reuse the script
* fix indentation in build.sh
1. Clarify that you need to manually create the user and group
when building from source. ./build.sh does not do that since
it is a packaging tool, not system configuration one.
2. Do not require "go" command to be present when running
./build.sh install. go installation may be user-specific and
unavailable when running with sudo.
3. Ease UMask restrictions. Allow group access.
This allows CLI commands to be run by any user in maddy group.
See #569.
Earlier in the `run` function, there was this code fragment:
```
package
export elevate=1
create_user
install_pkg
```
who always set the variable `elevate` to eq `1`, even if `sudo` is not installed on the user's system.
Let's not do this if user doesn't have `sudo` installed by adding more over-engineered code!
--longoptions is replaced with -l to stay compatible with busybox
getopt.
Options switch block was checking --buildversion by accident.
.git/ check was not running popd in case of failure.
New script build.sh is much more suitable for downstream packaging
(e.g. ./build.sh package) than hacked together package.sh wrapper for
get.sh while still being usable for "effort-less" installation.
Additionally, hostname setting in get.sh is flawed in many ways and is
not reimplemented in build.sh.
build.sh has proper command line options that allow to customize build
configuration and installation prefixes.
Documentation page get.sh is removed since all applicable environment
variables and flags are documented in ./build.sh --help.
build.sh can be called from the source directory to build maddy from
*this* source instead of forced 'go get' that was used in get.sh.
However, if build.sh is called not from the source directory, it clones
the repo and (optionally) uses the specified commit. This keeps build.sh
usable in curl|bash commands.
Due to the way source code is fetched, build.sh uses Git tags instead of
Go module versions as get.sh did.
