emerge --info: show /bin/sh provider (527996)

Searching contents for the /bin/sh provider is somewhat slow.
Therefore, use the basename of the symlink target to locate the
package. If this fails, then only the basename of the symlink target
will be displayed. So, typical output is something like
"sh bash 4.2_p53". Since realpath is used to resolve symlinks
recursively, this approach is also able to handle multiple levels of
symlinks such as /bin/sh -> bb -> busybox. Note that we do not parse
the output of "/bin/sh --version" because many shells do not have
a --version option.

The relevant section of the emerge --info output will now look
something like this:

Timestamp of tree: Fri, 28 Nov 2014 00:45:01 +0000
sh bash 4.2_p53
ld GNU ld (Gentoo 2.23.2 p1.0) 2.23.2

X-Gentoo-Bug: 527996
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=527996