Fix double apropos output when both alias name and command name match the pattern

[Philippe Waroquiers <philippe.waroquiers@skynet.be>]

apropos_cmd first tries to match the command name with the pattern.
If that succeeds, it prints the command and all its aliases not having
a specific doc string.
In this case, it should not try to match the command aliases names as
the command has already been printed.

Without this patch, the behaviour is:
(gdb) alias ter-exec = interpreter-exec
(gdb) apropos er-exe
interpreter-exec, ter-exec -- Execute a command in an interpreter.
interpreter-exec, ter-exec -- Execute a command in an interpreter.
(gdb)