Merge of r4948, r4949, r4964, r4973, r5011: variables.

Merge of r4948, r4949, r4964, r4973, r5011: variables.

*) Allow the complex value to be defined as an empty string.
   This makes conversion from strings to complex values possible
   without the loss of functionality.

*) The "auth_basic" directive gained support of variables.

*) Fixed variable syntax checking in "set", "geo", "limit_conn_zone",
   and "perl_set" directives.

*) Added checks that disallow adding a variable with an empty name.
   Added variable name syntax checks to "geo" and "map" directives.

*) Variables $pipe, $request_length, $time_iso8601, and $time_local.
   Log module counterparts are preserved for efficiency.
   Based on patch by Kiril Kalchev.