Add a number of abbrs and variants to \N{}

Add a number of abbrs and variants to \N{}

This patch adds the standard abbreviations for the control characters
(such as ACK, BEL, etc) to the repertoire that \N{} knows about.  It
also adds a few common variants of their full names, and the old names
for the 4 controls that Unicode has chosen not to have any names at all
for.

The patch also adds all the abbreviations that Unicode lists in 5.2 for
longer characters, such as NBSP, SHY, LRE, ...

To preserve complete backward compatibilty for these and future changes,
user-defined aliases are now checked first, before these are.

As a performance enhancement, these aliases are mapped to their actual
code values instead of their full names which then had to be looked up
in the large table.  Now that is avoided, and the table is not loaded
at all until a name is encountered that is not one of these aliases.

The pod and .t are updated.