Make the default PID file compile-time settable (#12485)

#12391 offered to change the default PID file from /var/run/gitea.pid however in discussion it was decided that this could break users of older systems. An alternative was offered that we could make the PID file compile/link time settable.

This PR does this, and changes the name of the setting from CustomPID to simply PIDFile. It also updates the from-source docs to show how to change the compiler settings to do this.

Closes #12391

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Florian Klink <flokli@flokli.de>

docs/content/doc/installation/from-source.en-us.md

@@ -155,6 +155,7 @@ using the `LDFLAGS` environment variable for `make`. The appropriate settings ar
 * For `CustomConf` you should use `-X \"code.gitea.io/gitea/modules/setting.CustomConf=conf.ini\"`
 * For `AppWorkPath` you should use `-X \"code.gitea.io/gitea/modules/setting.AppWorkPath=working-path\"`
 * For `StaticRootPath` you should use `-X \"code.gitea.io/gitea/modules/setting.StaticRootPath=static-root-path\"`
+* To change the default PID file location use `-X \"code.gitea.io/gitea/modules/setting.PIDFile=/run/gitea.pid\"`
 
 Add as many of the strings with their preceding `-X` to the `LDFLAGS` variable and run `make build`
 with the appropriate `TAGS` as above.

docs/content/doc/usage/command-line.en-us.md

@@ -44,7 +44,7 @@ Starts the server:
 - Examples:
     - `gitea web`
     - `gitea web --port 80`
-    - `gitea web --config /etc/gitea.ini --pid /var/run/gitea.pid`
+    - `gitea web --config /etc/gitea.ini --pid /some/custom/gitea.pid`
 - Notes:
     - Gitea should not be run as root. To bind to a port below 1024, you can use setcap on
       Linux: `sudo setcap 'cap_net_bind_service=+ep' /path/to/gitea`. This will need to be