* Support for pcre and pcre2 rewritten

The pcre2posix (or pcreposix) layer is no longer used.  Instead, pound
uses the native API of the corresponding library.  This provides for
additional speed-up and (in case of pcre2) avoids coredumps under high
load.

* Use of POSIX and Perl-compatible regular expressions

In contrast to previous versions, both types of regular expressions
can be used simultaneously in the configuration file.  The flavour
of the regex to use can be specified either individually with each
request matching statement (such as URL, Header, etc.), or globally.

To set it globally, use the RegexType statement.  The type of regular
expression set by it will be used in all matching statements that
appear below that statement, unless they declare regex type
explicitly.  It remains in effect until next RegexType statement or
end of file is encountered, whichever occurs first.  For example, to
use PCRE by default, add the following statement somewhere near the
beginning of your pound.cfg file:

  RegexType pcre

To change the default back to POSIX regexps, do

  RegexType posix

Regular expression type can also be specified with each matching
statement individually.  For example, the -pcre option indicates that
Perl-compatible regular expression is given as argument, e.g.:

  Host -pcre -icase "(?<!www\\.)example.org"

Similarly, the -posix option indicates that POSIX extended regular
expression is used.  Use these options to override the default for a
single statement.

* New matching option: -contain

The -contain option enables substring match. E.g. the following will
match if URL contains the word "user" (case-insensitive):

  URL -contain -icase "user"

* New configuration statement: LogTag

Sets a string to tag syslog messages with.  By default, the name used
to start the program is assumed.

* Bugfixes

** Fix infinite recursion on reporting an out of memory condition.
** Fix deadlock in session expiration handling.
** Fix RewriteLocation functionality.