Replace char * with something sensible

[Eli Zaretskii]

Rumor has it that latest versions of GCC whine about mixing "char *"
and "unsigned char *".  To fix that, Andrew committed some patches
that replace these with a "bfd_byte *", but I think we should not use
BFD data types in GDB unless they refer to data structures returned by
functions from the BFD library.

Possible solutions:

 . use "void *" (suggested by Mark Kettenis).

 . define a new data type gdb_byte and use "gdb_byte *".

I think Mark's suggestion should be considered first, since it avoids
the pain of introducing yet another custom data type.