src/common.{c,h}: Add to the repository.

Move common procedures for converting SCM values to SSH contants
and vice versa to common.c file.

* src/common.c, src/common.h: New files.
* src/message-func.c (_ssh_const_to_scm): Remove.
* src/server-func.c (guile_ssh_server_set_x): Use `_scm_to_ssh_const'.
* src/Makefile.am (libguile_ssh_la_SOURCES): Add common.{c,h}.