Backport #28717 (only the gitea.service sample)

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>

This PR adds support for the systemd notify protocol. Several status
messagess are provided. We should likely add a common notify/status
message for graceful.

Replaces #21140

Signed-off-by: Andrew Thornton <art27@cantab.net>

---------

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: ltdk <usr@ltdk.xyz>
Co-authored-by: Giteabot <teabot@gitea.io>

`LimitMEMLOCK=infinity` is already the default, so removed that

Default soft limit for LimitNOFILE is 1024
Default hard limit for LimitNOFILE is 524288

`LimitNOFILE=65535` increases the soft limit, but decreases the hard
limit

References:
* https://www.freedesktop.org/software/systemd/man/systemd.exec.html
*
https://blog.skbali.com/2019/09/set-ulimits-for-a-script-started-using-systemd/

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

* Comment on PrivateUsers option for gitea.service

A user happens to encounter an issue where PrivateUsers sandboxed Gitea.service and it effectively stop systemd from applying capabilities for that gitea.service. I am opening this PR to provide comments on PrivateUsers, effectively a tiny FAQ information for end-user.

`Requires=` has the behaviour of stopping `gitea.service` when the
database is stopped but not bringing it up again after the database is
started again. Use `Wants=` to define a weak requirement instead,
meaning `gitea.service` will be kept running when the database is
stopped, which is not an issue because gitea will just reconnect later
on.

Fixes: https://github.com/go-gitea/gitea/issues/15866

Co-authored-by: zeripath <art27@cantab.net>

If the gitea service is stopped because of the db going down
it needs an `After=db.service` to ensure it is restarted in
addition to the `Requires=db.service` to ensure that the db
is started before gitea is started.

Fix #15866

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

On some setups, Git is installed to directory prefix other than default
PATH (such as /opt/git/bin). For Gitea to know such Git installations,
PATH environment must be specified on service file.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>

Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

* rename socket file, make example more precise, use long parameter name

* correct path

* use the correct path to the configuration file according to the documentation

* rename the socket unit back to 'gitea.main.socket', explicitly indicate that the socket unit should use the service file 'gitea.service'

* Update the provided gitea.service since we have graceful restarting again

* Update contrib/systemd/gitea.service

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

When using unix socket as listener (`HTTP_ADDR = /run/gitea/gitea.socket`) then it's required to have the folder `/run/gitea` with appropriate owner/group. Manual creation leads to vanishing after reboot. This directive enables Systemd to handle this.

* More detailed documentation on how to set up from binary
Also change recommended file locations

in regards to binding to lower ports than 1024

* Renamed scripts directory into contrib

* Added script to download gitignores from github

* Synced gitignores with github repo