etc-update: symlink support for bug #485598

This includes numerous logic adjustments that are needed to support
protected symlinks. The show_diff function now supports arbitrary
file types. For example, a diff between two symlinks looks like this:

-SYM: /foo/bar -> baz
+SYM: /foo/bar -> blah

X-Gentoo-Bug: 485598
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=485598
dispatch-conf: symlink support for bug #485598

This includes numerous logic adjustments that are needed to support
protected symlinks. The new diff_mixed function is used for diffs
between arbitrary file types. For example, a diff between two symlinks
looks like this:

-SYM: /foo/bar -> baz
+SYM: /foo/bar -> blah

X-Gentoo-Bug: 485598
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=485598
CONFIG_PROTECT: protect symlinks, bug #485598

Users may not want some symlinks to get clobbered, so protect them
with CONFIG_PROTECT. Changes were required in the dblink.mergeme method
and the new_protect_filename function. The unit tests demonstrate
operation in many different scenarios. For example:

 * regular file replaces regular file
 * regular file replaces symlink
 * regular file replaces directory
 * symlink replaces symlink
 * symlink replaces regular file
 * symlink replaces directory
 * directory replaces regular file
 * directory replaces symlink

X-Gentoo-Bug: 485598
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=485598