RT1941: c_rehash.pod is missing

Add the file written by James Westby, graciously contributed
under the terms of the OpenSSL license.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(cherry picked from commit cf2239b3b3)

doc/apps/c_rehash.pod

@@ -0,0 +1,81 @@
+=pod
+
+=for comment
+Original text by James Westby, contributed under the OpenSSL license.
+
+=head1 NAME
+
+c_rehash - Create symbolic links to files named by the hash values
+
+=head1 SYNOPSIS
+
+B<c_rehash>
+[ I<directory>...]
+
+=head1 DESCRIPTION
+
+B<c_rehash> scans directories and calculates a hash value of each C<.pem>
+file in the specified directory list and creates symbolic links
+for each file, where the name of the link is the hash value.
+This utility is useful as many programs that use OpenSSL require
+directories to be set up like this in order to find certificates.
+
+If any directories are named on the command line, then those are
+processed in turn. If not, then the B<SSL_CERT_DIR> environment variable
+is consulted; this shold be a colon-separated list of directories,
+like the Unix B<PATH> variable.
+If that is not set then the default directory (installation-specific
+but often B</usr/local/ssl/certs>) is processed.
+
+In order for a directory to be processed, the user must have write
+permissions on that directory, otherwise it will be skipped.
+The links created are of the form C<HHHHHHHH.D>, where each B<H>
+is a hexadecimal character and B<D> is a single decimal digit.
+When processing a directory, B<c_rehash> will first remove all links
+that have a name in that syntax. If you have links in that format
+used for other purposes, they will be removed.
+Hashes for CRL's look similar except the letter B<r> appears after
+the period, like this: C<HHHHHHHH.rD>.
+
+Multiple objects may have the same hash; they will be indicated by
+incrementing the B<D> value. Duplicates are found by comparing the
+full SHA-1 fingerprint. A warning will be displayed if a duplicate
+is found.
+
+A warning will also be displayed if there are B<.pem> files that
+cannot be parsed as either a certificate or a CRL.
+
+The program uses the B<openssl> program to compute the hashes and
+fingerprints. If not found in the user's B<PATH>, then set the
+B<OPENSSL> environment variable to the full pathname.
+Any program can be used, it will be invoked as follows for either
+a certificate or CRL:
+
+  $OPENSSL x509 -hash -fingerprint -noout -in FFFFFF
+  $OPENSSL crl -hash -fingerprint -noout -in FFFFFF
+
+where B<FFFFFF> is the filename. It must output the hash of the
+file on the first line, and the fingerprint on the second,
+optionally prefixed with some text and an equals sign.
+
+=head1 ENVIRONMENT
+
+=over
+
+=item B<OPENSSL>
+
+The path to an executable to use to generate hashes and
+fingerprints (see above).
+
+=item B<SSL_CERT_DIR>
+
+Colon separated list of directories to operate on.
+Ignored if directories are listed on the command line.
+
+=back
+
+=head1 SEE ALSO
+
+L<openssl(1)|openssl(1)>,
+L<crl(1)|crl(1)>.
+L<x509(1)|x509(1)>.