Defined some feature identifiers specified in R7RS appendix B
Added srfi-106
Install srfi-106.scm
Updated the list of builtin modules
Added dummy srfi-*.scm for srfis provided in different gauche modules

For example, srfi-18 is provided by gauche.threads, but a portable
program may want say (import (srfi 18)) and we need to make them happy.
Autogenerate srfi/N.scm to make (import (srfi N)) work
Adapted gen-staticinit to deal with glob patterns in SCMFILES list