Switch to MUI2 and add Run after Install

MUI2 gives more flexibility in customizing pages.

• src/installer-options.ini.in: Removed.
• src/Makefile.am (installer-options.ini.in): Remove mentions. (g4wihelp.dll): Link with desktopshellrun.cpp
• src/g4wihelp.c: Fix implicit stdio usages.
• src/gpg4win-src.nsi, src/gpg4win.nsi (ABOUT_STR): MUI2 needs $\r$\n everywere. (Sections): Include Sections and Memento header. (installer.nsi): Include after sections.
• src/inst-sections.nsi, src/installer.nsi: Move generic functions into installer.nsi
• src/installer.nsi (FinishFunction): New. Conditional show run with correct Cert Manager. If not source installer. (RunAsUser): New. Use DesktopShellRun. (LICENSE_PAGE): Commented out. (MUI_UNIFINISHPAGE): Commented out.

The finish page now shows a Run GPA or Run Kleopatra depending
on wether or not they are selected. If none is selected nothing
is offered.

License and finish page for uninstaller and source installer removed.