Replace GNU RCS ident with a BSD license ident

Rationale: ident(1) is useful out of RCS, lot of scripts are using ident(1) and
failing when base is built WITHOUT_RCS.

This version is:
- fully compatible with RCS 5.7 ident.
- fully compatible with RCS 5.9 ident.
- passes all ident test from GNU RCS 5.9 test suite

This version has support for: svn extension for the Keyword id (double colon and
# before last $)

Différences with GNU RCS ident:
- no long options as found in GNU RCS 5.9 (but not commented there).
- '-V' reports nothing but has been added for compatibility.

Differential Revision: https://reviews.freebsd.org/D3200
Reviewed by: pfg