Correct error message in view_file

Used to assume broken symbolic link error, which may be incorrect.
Provide initial implementation of cmd-line editing
Decomposite get_and_execute_command in 6 functions
Reduce code duplication in normal.c and visual.c

Regarding handlers of /, ?, q/ and q? commands.
Return success code by running.c:view_file()

This allows to ignore commands, when external editor quits with non-zero
exit code.
Save initial line to history on editor failure
Document external command-line editing
Rename "global" to "glob" in documentation
Merge branch 'add-cmdline-edit'