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 changes to dblink.mergeme do 3 things:

 * Move the bulk of config protection logic from dblink.mergeme to a
   new dblink._protect method. The new method only returns 3 variables,
   which makes it easier to understand how config protection interacts
   with the dblink.mergeme code that uses those variables. This is
   important, since dblink.mergeme has so many variables.

 * Initialize more variables at the beginning of dblink.mergeme, since
   those variables are used by the dblink._protect method.

 * Use the variables returned from dblink._protect to trigger
   appropriate behavior later in dblink.mergeme.

The new_protect_filename changes are required since this function
compares the new file to old ._cfg* files that may already exist, in
order to avoid creating duplicate ._cfg* files. In these comparisons,
it needs to handle symlinks differently from regular files.

The unit tests demonstrate operation in many different scenarios,
including:

 * 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