Allow debugfs to be extended for use by test programs

Allow debugfs to be extended for use by test programs

This change allows debugfs to be reused as the base for e2fsprogs
internal test programs, by linking debugfs object files with
additional object file(s) that define additional commands.  The test
program's object file(s) should define their own comand table, and
define the symbol extra_cmds to be a pointer to the ss_request_table.
In addition, the symbol debug_prog_name can be used to override the
name of the program printed in the version banner and in the ss
prompt.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>