help: hide command line options marked as "advanced" Previously, we have keywords like "(DEPRECATED)" and "(EXPERIMENTAL)" to hide command line options in non-verbose help output. However, sometimes an option is neither deprecated nor experimental. It's well-tested and working, but just not designed to average users. This patch adds a keyword "(ADVANCED)" to fit in such use cases. Thanks rom1dep for the suggestion of the word "advanced".

pager: do not try to run an empty pager command If pagercmd is explicitly set to '', the pager process would exit silently and the output would be lost. We'd better disable the pager in such case.