Add ini and cmd line switch for browser

* doc/manual/einsteiger.tex: Slightly update ini example.
* src/gpg4win.nsi: Register new variable
* src/installer.nsi: Read new variable.
* src/inst-sections.nsi: Handle it.

The browser integration can now be toggled by adding
inst_browser = true to a gpg4win.ini or by adding
/WITH_BROWSER=1 to the command line when calling the installer.

• GnuPG-Bug-Id: T4297